diff --git a/src/library/CMakeLists.txt b/src/library/CMakeLists.txt
index dd3aeabba4..b79e0e1bb3 100644
--- a/src/library/CMakeLists.txt
+++ b/src/library/CMakeLists.txt
@@ -1,5 +1,5 @@
 add_library(library basic_thms.cpp deep_copy.cpp max_sharing.cpp
   printer.cpp formatter.cpp context_to_lambda.cpp
   state.cpp update_expr.cpp kernel_exception_formatter.cpp
-  reduce.cpp light_checker.cpp placeholder.cpp)
+  reduce.cpp light_checker.cpp placeholder.cpp ho_unifier.cpp)
 target_link_libraries(library ${LEAN_LIBS})
diff --git a/src/library/ho_unifier.cpp b/src/library/ho_unifier.cpp
new file mode 100644
index 0000000000..b3410d9610
--- /dev/null
+++ b/src/library/ho_unifier.cpp
@@ -0,0 +1,42 @@
+/*
+Copyright (c) 2013 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#include "kernel/environment.h"
+#include "library/ho_unifier.h"
+
+namespace lean {
+class ho_unifier::imp {
+    environment    m_env;
+    volatile bool  m_interrupted;
+
+public:
+    imp(environment const & env):m_env(env) {
+        m_interrupted = false;
+        // TODO(Leo)
+    }
+
+    bool unify(context const & ctx, expr const & l, expr const & r, metavar_env & menv, unification_problems & up) {
+        // TODO(Leo)
+        return false;
+    }
+
+    void clear() {
+        // TODO(Leo)
+    }
+
+    void set_interrupt(bool flag) {
+        m_interrupted = flag;
+    }
+};
+
+ho_unifier::ho_unifier(environment const & env):m_ptr(new imp(env)) {}
+ho_unifier::~ho_unifier() {}
+void ho_unifier::clear() { m_ptr->clear(); }
+void ho_unifier::set_interrupt(bool flag) { m_ptr->set_interrupt(flag); }
+bool ho_unifier::operator()(context const & ctx, expr const & l, expr const & r, metavar_env & menv, unification_problems & up) {
+    return m_ptr->unify(ctx, l, r, menv, up);
+}
+}
diff --git a/src/library/ho_unifier.h b/src/library/ho_unifier.h
new file mode 100644
index 0000000000..fe63011172
--- /dev/null
+++ b/src/library/ho_unifier.h
@@ -0,0 +1,43 @@
+/*
+Copyright (c) 2013 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#pragma once
+#include <memory>
+#include "kernel/metavar.h"
+
+namespace lean {
+/** \brief Functional object for (incomplete) higher-order unification */
+class ho_unifier {
+    class imp;
+    std::unique_ptr<imp> m_ptr;
+public:
+    ho_unifier(environment const & env);
+    ~ho_unifier();
+
+    /**
+        \brief Try to unify \c l and \c r in the context \c ctx using the substitution \c menv.
+        By unification, we mean we have to find an assignment for the unassigned metavariables in
+        \c l and \c r s.t. \c l and \c r become definitionally equal.
+        The unifier may produce a residue: a set of unification problems that could not be solved,
+        and were postponed. This set of problems is stored in \c up. The caller should try to instantiate
+        the metavariables in the residue using other constraints, and then try to continue the unification.
+        Return true if the unification succeeded (modulo residue), and false if the terms can't be unified.
+
+        @param[in] ctx The context for \c l and \c r
+        @param[in] l   Expression to be unified with \c r
+        @param[in] r   Expression to be unified with \c l
+        @param[in,out] menv  Metavariable substitution. \c menv may already contain some instantiated variables when this method is invoked.
+        @param[out]    up Delayed unification problems (aka residue), that could not be solved by the unifier.
+    */
+    bool operator()(context const & ctx, expr const & l, expr const & r, metavar_env & menv, unification_problems & up);
+
+    void clear();
+
+    void set_interrupt(bool flag);
+    void interrupt() { set_interrupt(true); }
+    void reset_interrupt() { set_interrupt(false); }
+};
+}