diff --git a/src/frontends/lean/decl_cmds.cpp b/src/frontends/lean/decl_cmds.cpp
index 56d2e66f0e..9f89b83788 100644
--- a/src/frontends/lean/decl_cmds.cpp
+++ b/src/frontends/lean/decl_cmds.cpp
@@ -8,6 +8,7 @@ Author: Leonardo de Moura
 #include "util/sstream.h"
 #include "kernel/type_checker.h"
 #include "kernel/abstract.h"
+#include "kernel/replace_fn.h"
 #include "kernel/for_each_fn.h"
 #include "library/scoped_ext.h"
 #include "library/aliases.h"
@@ -315,13 +316,87 @@ static bool is_curr_with_or_comma(parser & p) {
     return p.curr_is_token(get_with_tk()) || p.curr_is_token(get_comma_tk());
 }
 
-expr parse_equations(parser & p, name const & n, expr const & type, buffer<expr> & auxs) {
+/**
+   For convenience, the left-hand-side of a recursive equation may contain
+   undeclared variables.
+   We use parser::undef_id_to_local_scope to force the parser to create a local constant for
+   each undefined identifier.
+
+   This method validates occurrences of these variables. They can only occur as an application
+   or macro argument.
+*/
+static void validate_equation_lhs(parser const & p, expr const & lhs, buffer<expr> const & locals) {
+    if (is_app(lhs)) {
+        validate_equation_lhs(p, app_fn(lhs), locals);
+        validate_equation_lhs(p, app_arg(lhs), locals);
+    } else if (is_macro(lhs)) {
+        for (unsigned i = 0; i < macro_num_args(lhs); i++)
+            validate_equation_lhs(p, macro_arg(lhs, i), locals);
+    } else if (!is_local(lhs)) {
+        for_each(lhs, [&](expr const & e, unsigned) {
+                if (is_local(e) &&
+                    std::any_of(locals.begin(), locals.end(), [&](expr const & local) {
+                            return mlocal_name(e) == mlocal_name(local);
+                        })) {
+                    throw parser_error(sstream() << "invalid occurrence of variable '" << mlocal_name(lhs) <<
+                                       "' in the left-hand-side of recursive equation", p.pos_of(lhs));
+                }
+                return has_local(e);
+            });
+    }
+}
+
+/**
+   \brief Merge multiple occurrences of a variable in the left-hand-side of a recursive equation.
+
+   \see validate_equation_lhs
+*/
+static expr merge_equation_lhs_vars(expr const & lhs, buffer<expr> & locals) {
+    expr_map<expr> m;
+    unsigned j = 0;
+    for (unsigned i = 0; i < locals.size(); i++) {
+        unsigned k;
+        for (k = 0; k < i; k++) {
+            if (mlocal_name(locals[k]) == mlocal_name(locals[i])) {
+                m.insert(mk_pair(locals[i], locals[k]));
+                break;
+            }
+        }
+        if (k == i) {
+            locals[j] = locals[i];
+            j++;
+        }
+    }
+    if (j == locals.size())
+        return lhs;
+    locals.shrink(j);
+    return replace(lhs, [&](expr const & e) {
+            if (!has_local(e))
+                return some_expr(e);
+            if (is_local(e)) {
+                auto it = m.find(e);
+                if (it != m.end())
+                    return some_expr(it->second);
+            }
+            return none_expr();
+        });
+}
+
+static void throw_invalid_equation_lhs(name const & n, pos_info const & p) {
+    throw parser_error(sstream() << "invalid recursive equation, head symbol '"
+                       << n << "' in the left-hand-side does not correspond to function(s) being defined", p);
+}
+
+expr parse_equations(parser & p, name const & n, expr const & type, buffer<expr> & auxs,
+                     optional<local_environment> const & lenv, buffer<expr> const & ps) {
     buffer<expr> eqns;
+    buffer<expr> fns;
     {
-        parser::local_scope scope1(p);
-        parser::undef_id_to_local_scope scope2(p);
+        parser::local_scope scope1(p, lenv);
+        for (expr const & param : ps)
+            p.add_local(param);
         lean_assert(is_curr_with_or_comma(p));
-        expr f = mk_local(n, type);
+        fns.push_back(mk_local(n, type));
         if (p.curr_is_token(get_with_tk())) {
             while (p.curr_is_token(get_with_tk())) {
                 p.next();
@@ -330,28 +405,57 @@ expr parse_equations(parser & p, name const & n, expr const & type, buffer<expr>
                 expr g_type = p.parse_expr();
                 expr g      = mk_local(g_name, g_type);
                 auxs.push_back(g);
+                fns.push_back(g);
             }
         }
         p.check_token_next(get_comma_tk(), "invalid declaration, ',' expected");
-        p.add_local(f);
-        for (expr const & g : auxs)
-            p.add_local(g);
+        for (expr const & fn : fns)
+            p.add_local(fn);
         while (true) {
-            expr lhs = p.parse_expr();
+            expr lhs;
+            unsigned prev_num_undef_ids = p.get_num_undef_ids();
+            buffer<expr> locals;
+            {
+                parser::undef_id_to_local_scope scope2(p);
+                auto lhs_pos = p.pos();
+                lhs = p.parse_expr();
+                expr lhs_fn = get_app_fn(lhs);
+                if (is_explicit(lhs_fn))
+                    lhs_fn = get_explicit_arg(lhs_fn);
+                if (is_constant(lhs_fn))
+                    throw_invalid_equation_lhs(const_name(lhs_fn), lhs_pos);
+                if (is_local(lhs_fn) && std::all_of(fns.begin(), fns.end(), [&](expr const & fn) { return fn != lhs_fn; }))
+                    throw_invalid_equation_lhs(local_pp_name(lhs_fn), lhs_pos);
+                if (!is_local(lhs_fn))
+                    throw parser_error("invalid recursive equation, head symbol in left-hand-side is not a constant", lhs_pos);
+                unsigned num_undef_ids = p.get_num_undef_ids();
+                for (unsigned i = prev_num_undef_ids; i < num_undef_ids; i++) {
+                    locals.push_back(p.get_undef_id(i));
+                }
+            }
+            validate_equation_lhs(p, lhs, locals);
+            lhs = merge_equation_lhs_vars(lhs, locals);
             p.check_token_next(get_assign_tk(), "invalid declaration, ':=' expected");
-            expr rhs = p.parse_expr();
-            eqns.push_back(mk_equation(lhs, rhs));
+            {
+                parser::local_scope scope2(p);
+                for (expr const & local : locals)
+                    p.add_local(local);
+                expr rhs = p.parse_expr();
+                eqns.push_back(Fun(fns, Fun(locals, mk_equation(lhs, rhs), p)));
+            }
             if (!p.curr_is_token(get_comma_tk()))
                 break;
             p.next();
         }
     }
     if (p.curr_is_token(get_wf_tk())) {
+        auto pos = p.pos();
         p.next();
+        expr R   = p.save_pos(mk_expr_placeholder(), pos);
         expr Hwf = p.parse_expr();
-        return mk_equations(eqns.size(), eqns.data(), Hwf);
+        return mk_equations(fns.size(), eqns.size(), eqns.data(), R, Hwf);
     } else {
-        return mk_equations(eqns.size(), eqns.data());
+        return mk_equations(fns.size(), eqns.size(), eqns.data());
     }
 }
 
@@ -409,7 +513,7 @@ class definition_cmd_fn {
             auto pos = m_p.pos();
             m_type = m_p.parse_expr();
             if (is_curr_with_or_comma(m_p)) {
-                m_value = parse_equations(m_p, m_name, m_type, m_aux_decls);
+                m_value = parse_equations(m_p, m_name, m_type, m_aux_decls, optional<local_environment>(), buffer<expr>());
             } else if (!is_definition() && !m_p.curr_is_token(get_assign_tk())) {
                 check_end_of_theorem(m_p);
                 m_value = m_p.save_pos(mk_expr_placeholder(), pos);
@@ -427,7 +531,7 @@ class definition_cmd_fn {
                 m_p.next();
                 m_type = m_p.parse_scoped_expr(ps, *lenv);
                 if (is_curr_with_or_comma(m_p)) {
-                    m_value = parse_equations(m_p, m_name, m_type, m_aux_decls);
+                    m_value = parse_equations(m_p, m_name, m_type, m_aux_decls, lenv, ps);
                 } else if (!is_definition() && !m_p.curr_is_token(get_assign_tk())) {
                     check_end_of_theorem(m_p);
                     m_value = m_p.save_pos(mk_expr_placeholder(), pos);
diff --git a/src/frontends/lean/elaborator.cpp b/src/frontends/lean/elaborator.cpp
index 1cd707db85..1df7c78e22 100644
--- a/src/frontends/lean/elaborator.cpp
+++ b/src/frontends/lean/elaborator.cpp
@@ -33,6 +33,7 @@ Author: Leonardo de Moura
 #include "library/local_context.h"
 #include "library/tactic/expr_to_tactic.h"
 #include "library/error_handling/error_handling.h"
+#include "library/definitional/equations.h"
 #include "frontends/lean/local_decls.h"
 #include "frontends/lean/class.h"
 #include "frontends/lean/tactic_hint.h"
@@ -99,10 +100,11 @@ elaborator::elaborator(elaborator_context & ctx, name_generator const & ngen, bo
     m_has_sorry = has_sorry(m_ctx.m_env);
     m_relax_main_opaque = false;
     m_use_tactic_hints  = true;
-    m_no_info = false;
-    m_tc[0]  = mk_type_checker(ctx.m_env, m_ngen.mk_child(), false);
-    m_tc[1]  = mk_type_checker(ctx.m_env, m_ngen.mk_child(), true);
-    m_nice_mvar_names = nice_mvar_names;
+    m_no_info           = false;
+    m_in_equation_lhs   = false;
+    m_tc[0]             = mk_type_checker(ctx.m_env, m_ngen.mk_child(), false);
+    m_tc[1]             = mk_type_checker(ctx.m_env, m_ngen.mk_child(), true);
+    m_nice_mvar_names   = nice_mvar_names;
 }
 
 expr elaborator::mk_local(name const & n, expr const & t, binder_info const & bi) {
@@ -812,6 +814,155 @@ expr elaborator::visit_sorry(expr const & e) {
     return mk_app(update_constant(e, to_list(u)), m, e.get_tag());
 }
 
+expr const & elaborator::get_equation_fn(expr const & eq) const {
+    expr it = eq;
+    while (is_lambda(it))
+        it = binding_body(it);
+    if (!is_equation(it))
+        throw_elaborator_exception(env(), "ill-formed equation", eq);
+    expr const & fn = get_app_fn(equation_lhs(it));
+    if (!is_local(fn))
+        throw_elaborator_exception(env(), "ill-formed equation", eq);
+    return fn;
+}
+
+static expr copy_domain(unsigned num, expr const & source, expr const & target) {
+    if (num == 0) {
+        return target;
+    } else {
+        lean_assert(is_binding(source) && is_binding(target));
+        return update_binding(source, mk_as_is(binding_domain(source)), copy_domain(num-1, binding_body(source), binding_body(target)));
+    }
+}
+
+static constraint mk_equations_cnstr(environment const & env, io_state const & ios, expr const & m, expr const & eqns) {
+    justification j         = mk_failed_to_synthesize_jst(env, m);
+    auto choice_fn = [=](expr const & , expr const &, substitution const & s,
+                         name_generator const &) {
+        expr new_eqns = substitution(s).instantiate(eqns);
+        regular(env, ios) << "Equations:\n" << new_eqns << "\n\n";
+        // TODO(Leo);
+        return lazy_list<constraints>(constraints());
+    };
+    bool owner = true;
+    bool relax = false;
+    return mk_choice_cnstr(m, choice_fn, to_delay_factor(cnstr_group::MaxDelayed), owner, j, relax);
+}
+
+expr elaborator::visit_equations(expr const & eqns, constraint_seq & cs) {
+    buffer<expr> eqs;
+    buffer<expr> new_eqs;
+    optional<expr> new_R;
+    optional<expr> new_Hwf;
+
+    to_equations(eqns, eqs);
+
+    if (eqs.empty())
+        throw_elaborator_exception(env(), "invalid empty set of recursive equations", eqns);
+
+    if (is_wf_equations(eqns)) {
+        new_R   = visit(equations_wf_rel(eqns), cs);
+        new_Hwf = visit(equations_wf_proof(eqns), cs);
+        expr Hwf_type   = infer_type(*new_Hwf, cs);
+        expr wf         = visit(mk_constant("well_founded"), cs);
+        wf              = ::lean::mk_app(wf, *new_R);
+        justification j = mk_type_mismatch_jst(*new_Hwf, Hwf_type, wf, equations_wf_proof(eqns));
+        auto new_Hwf_cs = ensure_has_type(*new_Hwf, Hwf_type, wf, j, m_relax_main_opaque);
+        new_Hwf         = new_Hwf_cs.first;
+        cs             += new_Hwf_cs.second;
+    }
+
+    flet<optional<expr>> set1(m_equation_R, new_R);
+    unsigned num_fns = equations_num_fns(eqns);
+
+    optional<expr> first_eq;
+    for (expr const & eq : eqs) {
+        expr new_eq;
+        if (first_eq) {
+            // Replace first num_fns domains of eq with the ones in first_eq.
+            // This is a trick/hack to ensure the fns in each equation have
+            // the same elaborated type.
+            new_eq = visit(copy_domain(num_fns, *first_eq, eq), cs);
+        } else {
+            new_eq = visit(eq, cs);
+            first_eq = new_eq;
+        }
+        new_eqs.push_back(new_eq);
+    }
+
+    expr new_eqns;
+    if (new_R) {
+        new_eqns = mk_equations(num_fns, new_eqs.size(), new_eqs.data(), *new_R, *new_Hwf);
+    } else {
+        new_eqns = mk_equations(num_fns, new_eqs.size(), new_eqs.data());
+    }
+
+    lean_assert(first_eq && is_lambda(*first_eq));
+    expr type = binding_domain(*first_eq);
+    expr m = m_full_context.mk_meta(m_ngen, some_expr(type), eqns.get_tag());
+    register_meta(m);
+    constraint c = mk_equations_cnstr(env(), ios(), m, new_eqns);
+    cs += c;
+    return m;
+}
+
+expr elaborator::visit_equation(expr const & eq, constraint_seq & cs) {
+    expr const & lhs = equation_lhs(eq);
+    expr const & rhs = equation_rhs(eq);
+    expr lhs_fn = get_app_fn(lhs);
+    if (is_explicit(lhs_fn))
+        lhs_fn = get_explicit_arg(lhs_fn);
+    if (!is_local(lhs_fn))
+        throw exception("ill-formed equation");
+    expr new_lhs, new_rhs;
+    {
+        flet<bool> set(m_in_equation_lhs, true);
+        new_lhs = visit(lhs, cs);
+    }
+    {
+        optional<expr> some_new_lhs(new_lhs);
+        flet<optional<expr>> set1(m_equation_lhs, some_new_lhs);
+        new_rhs = visit(rhs, cs);
+    }
+
+    expr lhs_type = infer_type(new_lhs, cs);
+    expr rhs_type = infer_type(new_rhs, cs);
+    justification j = mk_justification(eq, [=](formatter const & fmt, substitution const & subst) {
+            substitution s(subst);
+            return pp_def_type_mismatch(fmt, local_pp_name(lhs_fn), s.instantiate(lhs_type), s.instantiate(rhs_type));
+        });
+    pair<expr, constraint_seq> new_rhs_cs = ensure_has_type(new_rhs, rhs_type, lhs_type, j, m_relax_main_opaque);
+    new_rhs = new_rhs_cs.first;
+    cs     += new_rhs_cs.second;
+    return mk_equation(new_lhs, new_rhs);
+}
+
+expr elaborator::visit_inaccessible(expr const & e, constraint_seq & cs) {
+    if (!m_in_equation_lhs)
+        throw_elaborator_exception(env(), "invalid occurrence of 'inaccessible' annotation, it must only occur in the "
+                                   "left-hand-side of recursive equations", e);
+    return mk_inaccessible(visit(get_annotation_arg(e), cs));
+}
+
+expr elaborator::visit_decreasing(expr const & e, constraint_seq & cs) {
+    if (!m_equation_lhs)
+        throw_elaborator_exception(env(), "invalid occurrence of 'decreasing' annotation, it must only occur in "
+                                   "the right-hand-side of recursive equations", e);
+    if (!m_equation_R)
+        throw_elaborator_exception(env(), "invalid occurrence of 'decreasing' annotation, it can only be used when "
+                                   "recursive equations are being defined by well-founded recursion", e);
+    expr const & lhs_fn = get_app_fn(*m_equation_lhs);
+    if (get_app_fn(decreasing_app(e)) != lhs_fn)
+        throw_elaborator_exception(env(), "invalid occurrence of 'decreasing' annotation, expression must be an "
+                                   "application of the recursive function being defined", e);
+    expr dec_app        = visit(decreasing_app(e), cs);
+    expr dec_proof      = visit(decreasing_proof(e), cs);
+    // Remark: perhaps we should enforce the type of dec_proof here.
+    // We may have enough information to wrap the arguments in a sigma type (reason: the type of the function being elaborated has holes).
+    // Possible solution: create a constraint that enforces the type as soon the type of function has been elaborated.
+    return mk_decreasing(dec_app, dec_proof);
+}
+
 expr elaborator::visit_core(expr const & e, constraint_seq & cs) {
     if (is_placeholder(e)) {
         return visit_placeholder(e, cs);
@@ -841,6 +992,14 @@ expr elaborator::visit_core(expr const & e, constraint_seq & cs) {
         return visit_core(get_explicit_arg(e), cs);
     } else if (is_sorry(e)) {
         return visit_sorry(e);
+    } else if (is_equations(e)) {
+        lean_unreachable();
+    } else if (is_equation(e)) {
+        return visit_equation(e, cs);
+    } else if (is_inaccessible(e)) {
+        return visit_inaccessible(e, cs);
+    } else if (is_decreasing(e)) {
+        return visit_decreasing(e, cs);
     } else {
         switch (e.kind()) {
         case expr_kind::Local:      return e;
@@ -882,6 +1041,8 @@ pair<expr, constraint_seq> elaborator::visit(expr const & e) {
         } else {
             r = visit_core(b, cs);
         }
+    } else if (is_equations(e)) {
+        r    = visit_equations(e, cs);
     } else if (is_explicit(get_app_fn(e))) {
         r    = visit_core(e, cs);
     } else {
diff --git a/src/frontends/lean/elaborator.h b/src/frontends/lean/elaborator.h
index 3196022c3e..6fae783fb5 100644
--- a/src/frontends/lean/elaborator.h
+++ b/src/frontends/lean/elaborator.h
@@ -53,6 +53,14 @@ class elaborator : public coercion_info_manager {
     // if m_no_info is true, we do not collect information when true,
     // we set is to true whenever we find no_info annotation.
     bool                 m_no_info;
+    // if m_in_equation_lhs is true, we are processing the left-hand-side of an equation
+    // and inaccessible expressions are allowed
+    bool                 m_in_equation_lhs;
+    // if m_equation_lhs is not none, we are processing the right-hand-side of an equation
+    // and decreasing expressions are allowed
+    optional<expr>       m_equation_lhs;
+    // if m_equation_R is not none when elaborator is processing recursive equation using the well-founded relation R.
+    optional<expr>       m_equation_R;
     bool                 m_use_tactic_hints;
     info_manager         m_pre_info_data;
     bool                 m_has_sorry;
@@ -151,6 +159,13 @@ class elaborator : public coercion_info_manager {
     std::tuple<expr, level_param_names> apply(substitution & s, expr const & e);
     pair<expr, constraints> elaborate_nested(list<expr> const & g, expr const & e,
                                              bool relax, bool use_tactic_hints, bool report_unassigned);
+
+    expr const & get_equation_fn(expr const & eq) const;
+    expr visit_equations(expr const & eqns, constraint_seq & cs);
+    expr visit_equation(expr const & e, constraint_seq & cs);
+    expr visit_inaccessible(expr const & e, constraint_seq & cs);
+    expr visit_decreasing(expr const & e, constraint_seq & cs);
+
 public:
     elaborator(elaborator_context & ctx, name_generator const & ngen, bool nice_mvar_names = false);
     std::tuple<expr, level_param_names> operator()(list<expr> const & ctx, expr const & e, bool _ensure_type,
diff --git a/src/frontends/lean/parser.cpp b/src/frontends/lean/parser.cpp
index ff51bb9db8..3569061f9a 100644
--- a/src/frontends/lean/parser.cpp
+++ b/src/frontends/lean/parser.cpp
@@ -76,6 +76,12 @@ parser::local_scope::local_scope(parser & p, environment const & env):
     m_p.m_env = env;
     m_p.push_local_scope();
 }
+parser::local_scope::local_scope(parser & p, optional<environment> const & env):
+    m_p(p), m_env(p.env()) {
+    if (env)
+        m_p.m_env = *env;
+    m_p.push_local_scope();
+}
 parser::local_scope::~local_scope() {
     m_p.pop_local_scope();
     m_p.m_env = m_env;
@@ -362,8 +368,11 @@ expr parser::propagate_levels(expr const & e, levels const & ls) {
     }
 }
 
-pos_info parser::pos_of(expr const & e, pos_info default_pos) {
-    if (auto it = m_pos_table.find(get_tag(e)))
+pos_info parser::pos_of(expr const & e, pos_info default_pos) const {
+    tag t = e.get_tag();
+    if (t == nulltag)
+        return default_pos;
+    if (auto it = m_pos_table.find(t))
         return *it;
     else
         return default_pos;
@@ -432,7 +441,7 @@ void parser::push_local_scope(bool save_options) {
     optional<options> opts;
     if (save_options)
         opts = m_ios.get_options();
-    m_parser_scope_stack = cons(parser_scope_stack_elem(opts, m_level_variables, m_variables, m_include_vars),
+    m_parser_scope_stack = cons(parser_scope_stack_elem(opts, m_level_variables, m_variables, m_include_vars, m_undef_ids.size()),
                                 m_parser_scope_stack);
 }
 
@@ -451,6 +460,7 @@ void parser::pop_local_scope() {
     m_level_variables    = s.m_level_variables;
     m_variables          = s.m_variables;
     m_include_vars       = s.m_include_vars;
+    m_undef_ids.shrink(s.m_num_undef_ids);
     m_parser_scope_stack = tail(m_parser_scope_stack);
 }
 
@@ -1111,7 +1121,9 @@ expr parser::id_to_expr(name const & id, pos_info const & p) {
         if (m_undef_id_behavior == undef_id_behavior::AssumeConstant) {
             r = save_pos(mk_constant(get_namespace(m_env) + id, ls), p);
         } else if (m_undef_id_behavior == undef_id_behavior::AssumeLocal) {
-            r = save_pos(mk_local(id, mk_expr_placeholder()), p);
+            expr local = mk_local(id, mk_expr_placeholder());
+            m_undef_ids.push_back(local);
+            r = save_pos(local, p);
         }
     }
     if (!r)
diff --git a/src/frontends/lean/parser.h b/src/frontends/lean/parser.h
index 86b6336c5c..9ae0350e2f 100644
--- a/src/frontends/lean/parser.h
+++ b/src/frontends/lean/parser.h
@@ -52,8 +52,10 @@ struct parser_scope_stack_elem {
     name_set           m_level_variables;
     name_set           m_variables;
     name_set           m_include_vars;
-    parser_scope_stack_elem(optional<options> const & o, name_set const & lvs, name_set const & vs, name_set const & ivs):
-        m_options(o), m_level_variables(lvs), m_variables(vs), m_include_vars(ivs) {}
+    unsigned           m_num_undef_ids;
+    parser_scope_stack_elem(optional<options> const & o, name_set const & lvs, name_set const & vs, name_set const & ivs,
+                            unsigned num_undef_ids):
+        m_options(o), m_level_variables(lvs), m_variables(vs), m_include_vars(ivs), m_num_undef_ids(num_undef_ids) {}
 };
 typedef list<parser_scope_stack_elem> parser_scope_stack;
 
@@ -130,6 +132,8 @@ class parser {
     // curr command token
     name                   m_cmd_token;
 
+    buffer<expr>           m_undef_ids;
+
     void display_warning_pos(unsigned line, unsigned pos);
     void display_warning_pos(pos_info p);
     void display_error_pos(unsigned line, unsigned pos);
@@ -255,8 +259,8 @@ public:
     pos_info pos() const { return pos_info(m_scanner.get_line(), m_scanner.get_pos()); }
     expr save_pos(expr e, pos_info p);
     expr rec_save_pos(expr const & e, pos_info p);
-    pos_info pos_of(expr const & e, pos_info default_pos);
-    pos_info pos_of(expr const & e) { return pos_of(e, pos()); }
+    pos_info pos_of(expr const & e, pos_info default_pos) const;
+    pos_info pos_of(expr const & e) const { return pos_of(e, pos()); }
     pos_info cmd_pos() const { return m_last_cmd_pos; }
     name const & get_cmd_token() const { return m_cmd_token; }
     void set_line(unsigned p) { return m_scanner.set_line(p); }
@@ -359,7 +363,10 @@ public:
     expr parse_scoped_expr(buffer<expr> const & ps, unsigned rbp = 0) { return parse_scoped_expr(ps.size(), ps.data(), rbp); }
 
     struct local_scope { parser & m_p; environment m_env;
-        local_scope(parser & p, bool save_options = false); local_scope(parser & p, environment const & env); ~local_scope();
+        local_scope(parser & p, bool save_options = false);
+        local_scope(parser & p, environment const & env);
+        local_scope(parser & p, optional<environment> const & env);
+        ~local_scope();
     };
     bool has_locals() const { return !m_local_decls.empty() || !m_local_level_decls.empty(); }
     void add_local_level(name const & n, level const & l, bool is_variable = false);
@@ -395,6 +402,11 @@ public:
     struct undef_id_to_const_scope : public flet<undef_id_behavior> { undef_id_to_const_scope(parser & p); };
     struct undef_id_to_local_scope : public flet<undef_id_behavior> { undef_id_to_local_scope(parser &); };
 
+    /** \brief Return the size of the stack of undefined local constants */
+    unsigned get_num_undef_ids() const { return m_undef_ids.size(); }
+    /** \brief Return the i-th undefined local constants */
+    expr const & get_undef_id(unsigned i) const { return m_undef_ids[i]; }
+
     /** \brief Elaborate \c e, and tolerate metavariables in the result. */
     std::tuple<expr, level_param_names> elaborate_relaxed(expr const & e, list<expr> const & ctx = list<expr>());
     /** \brief Elaborate \c e, and ensure it is a type. */
diff --git a/src/library/definitional/equations.cpp b/src/library/definitional/equations.cpp
index 3eae9603ff..8e23acc02e 100644
--- a/src/library/definitional/equations.cpp
+++ b/src/library/definitional/equations.cpp
@@ -22,18 +22,27 @@ static std::string * g_decreasing_opcode = nullptr;
 [[ noreturn ]] static void throw_eq_ex() { throw exception("unexpected occurrence of 'equation' expression"); }
 
 class equations_macro_cell : public macro_definition_cell {
+    unsigned m_num_fns;
 public:
+    equations_macro_cell(unsigned num_fns):m_num_fns(num_fns) {}
     virtual name get_name() const { return *g_equations_name; }
     virtual pair<expr, constraint_seq> get_type(expr const &, extension_context &) const { throw_eqs_ex(); }
     virtual optional<expr> expand(expr const &, extension_context &) const { throw_eqs_ex(); }
-    virtual void write(serializer & s) const { s.write_string(*g_equations_opcode); }
+    virtual void write(serializer & s) const { s << *g_equations_opcode << m_num_fns; }
+    unsigned get_num_fns() const { return m_num_fns; }
 };
 
 class equation_macro_cell : public macro_definition_cell {
 public:
     virtual name get_name() const { return *g_equation_name; }
-    virtual pair<expr, constraint_seq> get_type(expr const &, extension_context &) const { throw_eq_ex(); }
-    virtual optional<expr> expand(expr const &, extension_context &) const { throw_eq_ex(); }
+    virtual pair<expr, constraint_seq> get_type(expr const &, extension_context &) const {
+        expr dummy = mk_Prop();
+        return mk_pair(dummy, constraint_seq());
+    }
+    virtual optional<expr> expand(expr const &, extension_context &) const {
+        expr dummy = mk_Type();
+        return some_expr(dummy);
+    }
     virtual void write(serializer & s) const { s.write_string(*g_equation_opcode); }
 };
 
@@ -56,11 +65,18 @@ public:
     virtual void write(serializer & s) const { s.write_string(*g_decreasing_opcode); }
 };
 
-static macro_definition * g_equations  = nullptr;
 static macro_definition * g_equation   = nullptr;
 static macro_definition * g_decreasing = nullptr;
 
 bool is_equation(expr const & e) { return is_macro(e) && macro_def(e) == *g_equation; }
+
+bool is_lambda_equation(expr const & e) {
+    if (is_lambda(e))
+        return is_lambda_equation(binding_body(e));
+    else
+        return is_equation(e);
+}
+
 expr const & equation_lhs(expr const & e) { lean_assert(is_equation(e)); return macro_arg(e, 0); }
 expr const & equation_rhs(expr const & e) { lean_assert(is_equation(e)); return macro_arg(e, 1); }
 expr mk_equation(expr const & lhs, expr const & rhs) {
@@ -76,40 +92,54 @@ expr mk_decreasing(expr const & t, expr const & H) {
     return mk_macro(*g_decreasing, 2, args);
 }
 
-bool is_equations(expr const & e) { return is_macro(e) && macro_def(e) == *g_equations; }
+bool is_equations(expr const & e) { return is_macro(e) && macro_def(e).get_name() == *g_equations_name; }
 bool is_wf_equations_core(expr const & e) {
     lean_assert(is_equations(e));
-    return !is_equation(macro_arg(e, macro_num_args(e) - 1));
+    return macro_num_args(e) >= 3 && !is_lambda_equation(macro_arg(e, macro_num_args(e) - 1));
 }
 bool is_wf_equations(expr const & e) { return is_equations(e) && is_wf_equations_core(e); }
 unsigned equations_size(expr const & e) {
+    lean_assert(is_equations(e));
     if (is_wf_equations_core(e))
-        return macro_num_args(e) - 1;
+        return macro_num_args(e) - 2;
     else
         return macro_num_args(e);
 }
-void to_equations(expr const & e, buffer<expr> & eqns) {
-    lean_assert(is_equation(e));
-    unsigned sz = equations_size(e);
-    for (unsigned i = 0; i < sz; i++)
-        eqns.push_back(macro_arg(e, i));
+unsigned equations_num_fns(expr const & e) {
+    lean_assert(is_equations(e));
+    return static_cast<equations_macro_cell const*>(macro_def(e).raw())->get_num_fns();
 }
 expr const & equations_wf_proof(expr const & e) {
     lean_assert(is_wf_equations(e));
     return macro_arg(e, macro_num_args(e) - 1);
 }
-expr mk_equations(unsigned num, expr const * eqns) {
-    lean_assert(std::all_of(eqns, eqns+num, is_equation));
-    lean_assert(num > 0);
-    return mk_macro(*g_equations, num, eqns);
+expr const & equations_wf_rel(expr const & e) {
+    lean_assert(is_wf_equations(e));
+    return macro_arg(e, macro_num_args(e) - 2);
 }
-expr mk_equations(unsigned num, expr const * eqns, expr const & Hwf) {
-    lean_assert(std::all_of(eqns, eqns+num, is_equation));
-    lean_assert(num > 0);
+void to_equations(expr const & e, buffer<expr> & eqns) {
+    lean_assert(is_equations(e));
+    unsigned sz = equations_size(e);
+    for (unsigned i = 0; i < sz; i++)
+        eqns.push_back(macro_arg(e, i));
+}
+expr mk_equations(unsigned num_fns, unsigned num_eqs, expr const * eqs) {
+    lean_assert(num_fns > 0);
+    lean_assert(num_eqs > 0);
+    lean_assert(std::all_of(eqs, eqs+num_eqs, is_lambda_equation));
+    macro_definition def(new equations_macro_cell(num_fns));
+    return mk_macro(def, num_eqs, eqs);
+}
+expr mk_equations(unsigned num_fns, unsigned num_eqs, expr const * eqs, expr const & R, expr const & Hwf) {
+    lean_assert(num_fns > 0);
+    lean_assert(num_eqs > 0);
+    lean_assert(std::all_of(eqs, eqs+num_eqs, is_lambda_equation));
     buffer<expr> args;
-    args.append(num, eqns);
+    args.append(num_eqs, eqs);
+    args.push_back(R);
     args.push_back(Hwf);
-    return mk_macro(*g_equations, args.size(), args.data());
+    macro_definition def(new equations_macro_cell(num_fns));
+    return mk_macro(def, args.size(), args.data());
 }
 
 expr mk_inaccessible(expr const & e) { return mk_annotation(*g_inaccessible_name, e); }
@@ -120,7 +150,6 @@ void initialize_equations() {
     g_equation_name     = new name("equation");
     g_decreasing_name   = new name("decreasing");
     g_inaccessible_name = new name("innaccessible");
-    g_equations         = new macro_definition(new equations_macro_cell());
     g_equation          = new macro_definition(new equation_macro_cell());
     g_decreasing        = new macro_definition(new decreasing_macro_cell());
     g_equations_opcode  = new std::string("Eqns");
@@ -128,15 +157,17 @@ void initialize_equations() {
     g_decreasing_opcode = new std::string("Decr");
     register_annotation(*g_inaccessible_name);
     register_macro_deserializer(*g_equations_opcode,
-                                [](deserializer &, unsigned num, expr const * args) {
-                                    if (num == 0)
+                                [](deserializer & d, unsigned num, expr const * args) {
+                                    unsigned num_fns;
+                                    d >> num_fns;
+                                    if (num == 0 || num_fns == 0)
                                         throw corrupted_stream_exception();
-                                    if (!is_equation(args[num-1])) {
-                                        if (num == 1)
+                                    if (!is_lambda_equation(args[num-1])) {
+                                        if (num <= 2)
                                             throw corrupted_stream_exception();
-                                        return mk_equations(num-1, args, args[num-1]);
+                                        return mk_equations(num_fns, num-2, args, args[num-2], args[num-1]);
                                     } else {
-                                        return mk_equations(num, args);
+                                        return mk_equations(num_fns, num, args);
                                     }
                                 });
     register_macro_deserializer(*g_equation_opcode,
@@ -157,7 +188,6 @@ void finalize_equations() {
     delete g_equation_opcode;
     delete g_equations_opcode;
     delete g_decreasing_opcode;
-    delete g_equations;
     delete g_equation;
     delete g_decreasing;
     delete g_equations_name;
diff --git a/src/library/definitional/equations.h b/src/library/definitional/equations.h
index e6d0bbf70f..0c992d0263 100644
--- a/src/library/definitional/equations.h
+++ b/src/library/definitional/equations.h
@@ -12,6 +12,8 @@ bool is_equation(expr const & e);
 expr const & equation_lhs(expr const & e);
 expr const & equation_rhs(expr const & e);
 expr mk_equation(expr const & lhs, expr const & rhs);
+/** \brief Return true if e is of the form <tt>fun a_1 ... a_n, equation</tt> */
+bool is_lambda_equation(expr const & e);
 
 bool is_decreasing(expr const & e);
 expr const & decreasing_app(expr const & e);
@@ -21,10 +23,12 @@ expr mk_decreasing(expr const & t, expr const & H);
 bool is_equations(expr const & e);
 bool is_wf_equations(expr const & e);
 unsigned equations_size(expr const & e);
+unsigned equations_num_fns(expr const & e);
 void to_equations(expr const & e, buffer<expr> & eqns);
 expr const & equations_wf_proof(expr const & e);
-expr mk_equations(unsigned num, expr const * eqns);
-expr mk_equations(unsigned num, expr const * eqns, expr const & Hwf);
+expr const & equations_wf_rel(expr const & e);
+expr mk_equations(unsigned num_fns, unsigned num_eqs, expr const * eqs);
+expr mk_equations(unsigned num_fns, unsigned num_eqs, expr const * eqs, expr const & R, expr const & Hwf);
 
 expr mk_inaccessible(expr const & e);
 bool is_inaccessible(expr const & e);
diff --git a/tests/lean/extra/rec.lean b/tests/lean/extra/rec.lean
new file mode 100644
index 0000000000..da24a61b50
--- /dev/null
+++ b/tests/lean/extra/rec.lean
@@ -0,0 +1,31 @@
+import data.vector
+open nat vector
+
+check lt.base
+set_option pp.implicit true
+
+definition add : nat → nat → nat,
+add zero b     := b,
+add (succ a) b := succ (add a b)
+
+definition map {A B C : Type} (f : A → B → C) : Π {n}, vector A n → vector B n → vector C n,
+map nil nil             := nil,
+map (a :: va) (b :: vb) := f a b :: map va vb
+
+definition fib : nat → nat,
+fib 0     := 1,
+fib 1     := 1,
+fib (a+2) := (fib a ↓ lt.step (lt.base a)) + (fib (a+1) ↓ lt.base (a+1))
+[wf] lt.wf
+
+definition half : nat → nat,
+half 0     := 0,
+half 1     := 0,
+half (x+2) := half x + 1
+
+variables {A B : Type}
+inductive image_of (f : A → B) : B → Type :=
+mk : Π a, image_of f (f a)
+
+definition inv {f : A → B} : Π b, image_of f b → A,
+inv ⌞f a⌟ (image_of.mk f a) := a