diff --git a/src/builtin/CMakeLists.txt b/src/builtin/CMakeLists.txt
index 36ff51bfbf..17822f71b1 100644
--- a/src/builtin/CMakeLists.txt
+++ b/src/builtin/CMakeLists.txt
@@ -95,6 +95,7 @@ add_theory("Real.lean"         "${CMAKE_CURRENT_BINARY_DIR}/Int.olean")
 add_theory("specialfn.lean"    "${CMAKE_CURRENT_BINARY_DIR}/Real.olean")
 add_theory("subtype.lean"      "${CMAKE_CURRENT_BINARY_DIR}/kernel.olean")
 add_theory("optional.lean"     "${CMAKE_CURRENT_BINARY_DIR}/subtype.olean")
+add_theory("sum.lean"          "${CMAKE_CURRENT_BINARY_DIR}/optional.olean")
 
 update_interface("kernel.olean"       "kernel" "-n")
 update_interface("Nat.olean"          "library/arith" "-n")
diff --git a/src/builtin/obj/kernel.olean b/src/builtin/obj/kernel.olean
index 6c1b816f47..9b1671905d 100644
Binary files a/src/builtin/obj/kernel.olean and b/src/builtin/obj/kernel.olean differ
diff --git a/src/builtin/obj/optional.olean b/src/builtin/obj/optional.olean
index c4e092fe2b..ab2fa979ca 100644
Binary files a/src/builtin/obj/optional.olean and b/src/builtin/obj/optional.olean differ
diff --git a/src/builtin/obj/subtype.olean b/src/builtin/obj/subtype.olean
index c4ee00afdf..a408f48c2c 100644
Binary files a/src/builtin/obj/subtype.olean and b/src/builtin/obj/subtype.olean differ
diff --git a/src/builtin/obj/sum.olean b/src/builtin/obj/sum.olean
new file mode 100644
index 0000000000..88a9321beb
Binary files /dev/null and b/src/builtin/obj/sum.olean differ
diff --git a/src/builtin/optional.lean b/src/builtin/optional.lean
index fcb0cb3705..ba1a299274 100644
--- a/src/builtin/optional.lean
+++ b/src/builtin/optional.lean
@@ -20,25 +20,25 @@ theorem some_pred {A : (Type U)} (a : A) : optional_pred A (λ x, x = a)
 theorem none_pred (A : (Type U)) : optional_pred A (λ x, false)
 := or_introl (take x, not_false_trivial) (exists_unique (λ x, false))
 
-theorem optional_inhabited (A : (Type U)) : inhabited (optional A)
+theorem inhab (A : (Type U)) : inhabited (optional A)
 := subtype_inhabited (exists_intro (λ x, false) (none_pred A))
 
 definition none {A : (Type U)} : optional A
-:= abst (λ x, false) (optional_inhabited A)
+:= abst (λ x, false) (inhab A)
 
 definition some {A : (Type U)} (a : A) : optional A
-:= abst (λ x, x = a) (optional_inhabited A)
+:= abst (λ x, x = a) (inhab A)
 
 definition is_some {A : (Type U)} (n : optional A) := ∃ x : A, some x = n
 
 theorem injectivity {A : (Type U)} {a a' : A} : some a = some a' → a = a'
 := assume Heq,
-     let eq_reps : (λ x, x = a) = (λ x, x = a') := abst_inj (optional_inhabited A) (some_pred a) (some_pred a') Heq
+     let eq_reps : (λ x, x = a) = (λ x, x = a') := abst_inj (inhab A) (some_pred a) (some_pred a') Heq
      in  (congr1 a eq_reps) ◂ (refl a)
 
 theorem distinct {A : (Type U)} (a : A) : some a ≠ none
 := assume N : some a = none,
-     let eq_reps : (λ x, x = a) = (λ x, false) := abst_inj (optional_inhabited A) (some_pred a) (none_pred A) N
+     let eq_reps : (λ x, x = a) = (λ x, false) := abst_inj (inhab A) (some_pred a) (none_pred A) N
      in (congr1 a eq_reps) ◂ (refl a)
 
 definition value {A : (Type U)} (n : optional A) (H : is_some n) : A
@@ -69,13 +69,13 @@ theorem dichotomy {A : (Type U)} (n : optional A) : n = none ∨ ∃ a, n = some
 := let pred : optional_pred A (rep n) :=  P_rep n
    in or_elim pred
         (λ Hl, let rep_n_eq     : rep n    = λ x, false    := false_pred Hl,
-                   rep_none_eq  : rep none = λ x, false    := rep_abst (optional_inhabited A) (λ x, false) (none_pred A)
+                   rep_none_eq  : rep none = λ x, false    := rep_abst (inhab A) (λ x, false) (none_pred A)
                in  or_introl (rep_inj (trans rep_n_eq (symm rep_none_eq)))
                              (∃ a, n = some a))
         (λ Hr : ∃ x, rep n x ∧ ∀ y, y ≠ x → ¬ rep n y,
            obtain (w : A) (Hw : rep n w ∧ ∀ y, y ≠ w → ¬ rep n y), from Hr,
               let rep_n_eq    : rep n        = λ x, x = w   := singleton_pred Hw,
-                  rep_some_eq : rep (some w) = λ x, x = w   := rep_abst (optional_inhabited A) (λ x, x = w) (some_pred w),
+                  rep_some_eq : rep (some w) = λ x, x = w   := rep_abst (inhab A) (λ x, x = w) (some_pred w),
                   n_eq_some   : n = some w                  := rep_inj (trans rep_n_eq (symm rep_some_eq))
               in or_intror (n = none)
                            (exists_intro w n_eq_some))
@@ -96,5 +96,5 @@ end
 
 set_opaque optional       true
 set_opaque optional_pred  true
-definition optional_inhabited := optional::optional_inhabited
+definition optional_inhabited := optional::inhab
 add_rewrite optional::is_some_some optional::not_is_some_none optional::distinct optional::value_some
diff --git a/src/builtin/sum.lean b/src/builtin/sum.lean
new file mode 100644
index 0000000000..46aa6f63ef
--- /dev/null
+++ b/src/builtin/sum.lean
@@ -0,0 +1,122 @@
+-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+-- Released under Apache 2.0 license as described in the file LICENSE.
+-- Author: Leonardo de Moura
+import macros
+import subtype
+import optional
+using subtype
+using optional
+-- We are encoding the (sum A B) as a subtype of (optional A) ⨯ (optional B), where
+--    (proj1 n = none) ≠ (proj2 n = none)
+definition sum_pred (A B : (Type U)) := λ p : (optional A) ⨯ (optional B), (proj1 p = none) ≠ (proj2 p = none)
+definition sum (A B : (Type U)) := subtype ((optional A) ⨯ (optional B)) (sum_pred A B)
+
+namespace sum
+-- TODO: move pair, pair_inj1 and pair_inj2 to separate file
+definition pair {A : (Type U)} {B : (Type U)} (a : A) (b : B) := tuple a, b
+theorem    pair_inj1 {A : (Type U)} {B : A → (Type U)} {a b : sig x, B x} (H : a = b) : proj1 a = proj1 b
+:= subst (refl (proj1 a)) H
+theorem    pair_inj2 {A B : (Type U)} {a b : A ⨯ B} (H : a = b) : proj2 a = proj2 b
+:= subst (refl (proj2 a)) H
+theorem    pairext_proj {A B : (Type U)} {p : A ⨯ B} {a : A} {b : B} (H1 : proj1 p = a) (H2 : proj2 p = b) : p = (pair a b)
+:= @pairext A (λ x, B) p (pair a b) H1 (to_heq H2)
+
+theorem inl_pred {A : (Type U)} (a : A) (B : (Type U)) : sum_pred A B (pair (some a) none)
+:= not_intro
+     (λ N : (some a = none) = (none = (optional::@none B)),
+        let eq  :   some a = none := (symm N) ◂ (refl (optional::@none B))
+        in absurd eq (distinct a))
+
+theorem inr_pred (A : (Type U)) {B : (Type U)} (b : B) : sum_pred A B (pair none (some b))
+:= not_intro
+     (λ N : (none = (optional::@none A)) = (some b = none),
+        let eq  :   some b = none := N ◂ (refl (optional::@none A))
+        in absurd eq (distinct b))
+
+theorem inhabl {A : (Type U)} (H : inhabited A) (B : (Type U)) : inhabited (sum A B)
+:= inhabited_elim H (λ w : A,
+      subtype_inhabited (exists_intro (pair (some w) none) (inl_pred w B)))
+
+theorem inhabr (A : (Type U)) {B : (Type U)} (H : inhabited B) : inhabited (sum A B)
+:= inhabited_elim H (λ w : B,
+      subtype_inhabited (exists_intro (pair none (some w)) (inr_pred A w)))
+
+definition inl {A : (Type U)} (a : A) (B : (Type U)) : sum A B
+:= abst (pair (some a) none) (inhabl (inhabited_intro a) B)
+
+definition inr (A : (Type U)) {B : (Type U)} (b : B) : sum A B
+:= abst (pair none (some b)) (inhabr A (inhabited_intro b))
+
+theorem inl_inj {A : (Type U)} (a1 a2 : A) (B : (Type U)) : inl a1 B = inl a2 B → a1 = a2
+:= assume Heq : inl a1 B = inl a2 B,
+     let eq1    : inl a1 B = abst (pair (some a1) none) (inhabl (inhabited_intro a1) B) := refl (inl a1 B),
+         eq2    : inl a2 B = abst (pair (some a2) none) (inhabl (inhabited_intro a1) B)
+                := subst (refl (inl a2 B)) (proof_irrel (inhabl (inhabited_intro a2) B) (inhabl (inhabited_intro a1) B)),
+         rep_eq : (pair (some a1) none) = (pair (some a2) none)
+                := abst_inj (inhabl (inhabited_intro a1) B) (inl_pred a1 B) (inl_pred a2 B) (trans (trans (symm eq1) Heq) eq2)
+     in optional::injectivity
+            (calc some a1 = proj1 (pair (some a1) (optional::@none B))  : refl (some a1)
+                      ... = proj1 (pair (some a2) (optional::@none B))  : pair_inj1 rep_eq
+                      ... = some a2                                     : refl (some a2))
+
+theorem inr_inj (A : (Type U)) {B : (Type U)} (b1 b2 : B) : inr A b1 = inr A b2 → b1 = b2
+:= assume Heq : inr A b1   = inr A b2,
+     let eq1    : inr A b1 = abst (pair none (some b1)) (inhabr A (inhabited_intro b1)) := refl (inr A b1),
+         eq2    : inr A b2 = abst (pair none (some b2)) (inhabr A (inhabited_intro b1))
+                := subst (refl (inr A b2)) (proof_irrel (inhabr A (inhabited_intro b2)) (inhabr A (inhabited_intro b1))),
+         rep_eq : (pair none (some b1)) = (pair none (some b2))
+                := abst_inj (inhabr A (inhabited_intro b1)) (inr_pred A b1) (inr_pred A b2) (trans (trans (symm eq1) Heq) eq2)
+     in optional::injectivity
+            (calc some b1 = proj2 (pair (optional::@none A) (some b1))  : refl (some b1)
+                      ... = proj2 (pair (optional::@none A) (some b2))  : pair_inj2 rep_eq
+                      ... = some b2                                     : refl (some b2))
+
+theorem distinct {A B : (Type U)} (a : A) (b : B) : inl a B ≠ inr A b
+:= assume N : inl a B = inr A b,
+     let eq1    : inl a B = abst (pair (some a) none) (inhabl (inhabited_intro a) B) := refl (inl a B),
+         eq2    : inr A b = abst (pair none (some b)) (inhabl (inhabited_intro a) B)
+                := subst (refl (inr A b)) (proof_irrel (inhabr A (inhabited_intro b)) (inhabl (inhabited_intro a) B)),
+         rep_eq : (pair (some a) none) = (pair none (some b))
+                := abst_inj (inhabl (inhabited_intro a) B) (inl_pred a B) (inr_pred A b) (trans (trans (symm eq1) N) eq2)
+     in absurd (pair_inj1 rep_eq) (optional::distinct a)
+
+theorem dichotomy {A B : (Type U)} (n : sum A B) : (∃ a, n = inl a B) ∨ (∃ b, n = inr A b)
+:= let pred : (proj1 (rep n) = none) ≠ (proj2 (rep n) = none) := P_rep n
+   in or_elim (em (proj1 (rep n) = none))
+         (λ Heq,  let neq_none : ¬ proj2 (rep n) = (optional::@none B) := (symm (not_iff_elim (ne_symm pred))) ◂ Heq,
+                      ex_some  : ∃ b, proj2 (rep n) = some b          := resolve1 (optional::dichotomy (proj2 (rep n))) neq_none
+                  in obtain (b : B) (Hb : proj2 (rep n) = some b), from ex_some,
+                     or_intror (∃ a, n = inl a B)
+                               (let rep_eq   : rep n         = pair none (some b)
+                                             := pairext_proj Heq Hb,
+                                    rep_inr  : rep (inr A b) = pair none (some b)
+                                             := rep_abst (inhabr A (inhabited_intro b)) (pair none (some b)) (inr_pred A b),
+                                    n_eq_inr : n = inr A b
+                                             := rep_inj (trans rep_eq (symm rep_inr))
+                                in exists_intro b n_eq_inr))
+         (λ Hne, let eq_none : proj2 (rep n) = (optional::@none B) := (not_iff_elim pred) ◂ Hne,
+                     ex_some : ∃ a, proj1 (rep n) = some a := resolve1 (optional::dichotomy (proj1 (rep n))) Hne
+                 in obtain (a : A) (Ha : proj1 (rep n) = some a), from ex_some,
+                    or_introl (let rep_eq   : rep n  = pair (some a) none
+                                            := pairext_proj Ha eq_none,
+                                   rep_inl  : rep (inl a B) = pair (some a) none
+                                            := rep_abst (inhabl (inhabited_intro a) B) (pair (some a) none) (inl_pred a B),
+                                   n_eq_inl : n = inl a B
+                                            := rep_inj (trans rep_eq (symm rep_inl))
+                               in exists_intro a n_eq_inl)
+                              (∃ b, n = inr A b))
+
+theorem induction {A B : (Type U)} {P : sum A B → Bool} (H1 : ∀ a, P (inl a B)) (H2 : ∀ b, P (inr A b)) : ∀ n, P n
+:= take n, or_elim (sum::dichotomy n)
+    (λ Hex : ∃ a, n = inl a B,
+        obtain (a : A) (Ha : n = inl a B), from Hex,
+          subst (H1 a) (symm Ha))
+    (λ Hex : ∃ b, n = inr A b,
+        obtain (b : B) (Hb : n = inr A b), from Hex,
+           subst (H2 b) (symm Hb))
+
+set_opaque inl true
+set_opaque inr true
+end
+set_opaque sum_pred true
+set_opaque sum      true
diff --git a/src/library/elaborator/elaborator.cpp b/src/library/elaborator/elaborator.cpp
index ae2bfc763c..7db68e73ff 100644
--- a/src/library/elaborator/elaborator.cpp
+++ b/src/library/elaborator/elaborator.cpp
@@ -1522,8 +1522,6 @@ class elaborator::imp {
             }
         }
 
-        if (!is_meta_app(a) && !is_meta_app(b) && normalize_head(a, b, c)) { return true; }
-
         if (!eq) {
             // Try to assign convertability constraints.
             if (is_metavar(a) && !is_assigned(a) && !has_local_context(a)) {
@@ -1549,6 +1547,8 @@ class elaborator::imp {
             }
         }
 
+        if (!is_meta_app(a) && !is_meta_app(b) && normalize_head(a, b, c)) { return true; }
+
         if (process_simple_ho_match(ctx, a, b, true, c) ||
             process_simple_ho_match(ctx, b, a, false, c))
             return true;