diff --git a/src/library/CMakeLists.txt b/src/library/CMakeLists.txt
index 8f916bc25..46179eb56 100644
--- a/src/library/CMakeLists.txt
+++ b/src/library/CMakeLists.txt
@@ -3,7 +3,8 @@ add_library(library deep_copy.cpp expr_lt.cpp io_state.cpp occurs.cpp
   resolve_macro.cpp kernel_serializer.cpp max_sharing.cpp
   normalize.cpp shared_environment.cpp module.cpp coercion.cpp
   private.cpp placeholder.cpp aliases.cpp level_names.cpp
-  update_declaration.cpp choice.cpp scoped_ext.cpp locals.cpp)
+  update_declaration.cpp choice.cpp scoped_ext.cpp locals.cpp
+  unifier.cpp)
 # hop_match.cpp)
 
 target_link_libraries(library ${LEAN_LIBS})
diff --git a/src/library/unifier.cpp b/src/library/unifier.cpp
new file mode 100644
index 000000000..a502f4903
--- /dev/null
+++ b/src/library/unifier.cpp
@@ -0,0 +1,346 @@
+/*
+Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#include <utility>
+#include "util/luaref.h"
+#include "util/lazy_list_fn.h"
+#include "kernel/for_each_fn.h"
+#include "kernel/abstract.h"
+#include "kernel/type_checker.h"
+#include "library/unifier.h"
+#include "library/kernel_bindings.h"
+
+namespace lean {
+static std::pair<unify_status, substitution> unify_simple_core(substitution const & s, expr const & lhs, expr const & rhs,
+                                                               justification const & j) {
+    lean_assert(is_meta(lhs));
+    buffer<expr> args;
+    expr const & m = get_app_args(lhs, args);
+    lean_assert(is_metavar(m));
+    for (auto it = args.begin(); it != args.end(); it++) {
+        if (!is_local(*it) || std::find(args.begin(), it, *it) != it)
+            return mk_pair(unify_status::Unsupported, s);
+    }
+    if (is_meta(rhs) && get_app_fn(rhs) == m)
+        return mk_pair(unify_status::Unsupported, s);
+    bool failed = false;
+    for_each(rhs, [&](expr const & e, unsigned) {
+            if (failed)
+                return false;
+            if (is_local(e) && std::find(args.begin(), args.end(), e) == args.end()) {
+                // right-hand-side contains variable that is not in the scope
+                // of metavariable.
+                failed = true;
+                return false;
+            }
+            if (is_metavar(e) && e == m) {
+                // occurs-check failed
+                failed = true;
+                return false;
+            }
+            // we only need to continue exploring e if it contains
+            // metavariables and/or local constants.
+            return has_metavar(e) || has_local(e);
+        });
+    if (failed)
+        return mk_pair(unify_status::Failed, s);
+    expr v = abstract_locals(rhs, args.size(), args.data());
+    unsigned i = args.size();
+    while (i > 0) {
+        --i;
+        v = mk_lambda(local_pp_name(args[i]), mlocal_type(args[i]), v);
+    }
+    return mk_pair(unify_status::Solved, s.assign(mlocal_name(m), v, j));
+}
+
+std::pair<unify_status, substitution> unify_simple(substitution const & s, expr const & lhs, expr const & rhs, justification const & j) {
+    if (lhs == rhs)
+        return mk_pair(unify_status::Solved, s);
+    else if (!has_metavar(lhs) && !has_metavar(rhs))
+        return mk_pair(unify_status::Failed, s);
+    else if (is_meta(lhs))
+        return unify_simple_core(s, lhs, rhs, j);
+    else if (is_meta(rhs))
+        return unify_simple_core(s, rhs, lhs, j);
+    else
+        return mk_pair(unify_status::Unsupported, s);
+}
+
+std::pair<unify_status, substitution> unify_simple_core(substitution const & s, level const & lhs, level const & rhs, justification const & j) {
+    lean_assert(is_meta(lhs));
+    bool contains = false;
+    for_each(rhs, [&](level const & l) {
+            if (contains)
+                return false;
+            if (l == lhs) {
+                // occurs-check failed
+                contains = true;
+                return false;
+            }
+            return true;
+        });
+    if (contains) {
+        if (is_succ(rhs))
+            return mk_pair(unify_status::Failed, s);
+        else
+            return mk_pair(unify_status::Unsupported, s);
+    }
+    return mk_pair(unify_status::Solved, s.assign(meta_id(lhs), rhs, j));
+}
+
+std::pair<unify_status, substitution> unify_simple(substitution const & s, level const & lhs, level const & rhs, justification const & j) {
+    if (lhs == rhs)
+        return mk_pair(unify_status::Solved, s);
+    else if (!has_meta(lhs) && !has_meta(rhs))
+        return mk_pair(unify_status::Failed, s);
+    else if (is_meta(lhs))
+        return unify_simple_core(s, lhs, rhs, j);
+    else if (is_meta(rhs))
+        return unify_simple_core(s, rhs, lhs, j);
+    else if (is_succ(lhs) && is_succ(rhs))
+        return unify_simple(s, succ_of(lhs), succ_of(rhs), j);
+    else
+        return mk_pair(unify_status::Unsupported, s);
+}
+
+std::pair<unify_status, substitution> unify_simple(substitution const & s, constraint const & c) {
+    if (is_eq_cnstr(c))
+        return unify_simple(s, cnstr_lhs_expr(c), cnstr_rhs_expr(c), c.get_justification());
+    else if (is_level_cnstr(c))
+        return unify_simple(s, cnstr_lhs_level(c), cnstr_rhs_level(c), c.get_justification());
+    else
+        return mk_pair(unify_status::Unsupported, s);
+}
+
+struct unifier_fn {
+    environment    m_env;
+    name_generator m_ngen;
+    substitution   m_subst;
+    unifier_plugin m_plugin;
+    bool           m_use_exception;
+
+    unifier_fn(environment const & env, unsigned /* num_cs */, constraint const * /* cs */,
+               name_generator const & ngen, substitution const & s, unifier_plugin const & p,
+               bool use_exception):
+        m_env(env), m_ngen(ngen), m_subst(s), m_plugin(p), m_use_exception(use_exception) {
+    }
+
+    optional<substitution> next() {
+        // TODO(Leo)
+        return optional<substitution>();
+    }
+};
+
+lazy_list<substitution> unify(std::shared_ptr<unifier_fn> const & u) {
+    return mk_lazy_list<substitution>([=]() {
+            auto s = u->next();
+            if (s)
+                return some(mk_pair(*s, unify(u)));
+            else
+                return lazy_list<substitution>::maybe_pair();
+        });
+}
+
+lazy_list<substitution> unify(environment const & env,  unsigned num_cs, constraint const * cs, name_generator const & ngen,
+                              unifier_plugin const & p, bool use_exception) {
+    return unify(std::make_shared<unifier_fn>(env, num_cs, cs, ngen, substitution(), p, use_exception));
+}
+
+lazy_list<substitution> unify(environment const & env, unsigned num_cs, constraint const * cs, name_generator const & ngen,
+                              bool use_exception) {
+    return unify(env, num_cs, cs, ngen, [](constraint const &, name_generator const &) { return lazy_list<constraints>(); }, use_exception);
+}
+
+lazy_list<substitution> unify(environment const & env, expr const & lhs, expr const & rhs, name_generator const & ngen, unifier_plugin const & p) {
+    substitution s;
+    buffer<constraint> cs;
+    name_generator new_ngen(ngen);
+    bool failed = false;
+    type_checker tc(env, new_ngen.mk_child(), [&](constraint const & c) {
+            if (!failed) {
+                auto r = unify_simple(s, c);
+                switch (r.first) {
+                case unify_status::Solved:
+                    s = r.second; break;
+                case unify_status::Failed:
+                    failed = true; break;
+                case unify_status::Unsupported:
+                    cs.push_back(c); break;
+                }
+            }
+        });
+    if (!tc.is_def_eq(lhs, rhs) || failed) {
+        return lazy_list<substitution>();
+    } else if (cs.empty()) {
+        return lazy_list<substitution>(s);
+    } else {
+        return unify(std::make_shared<unifier_fn>(env, cs.size(), cs.data(), ngen, s, p, false));
+    }
+}
+
+lazy_list<substitution> unify(environment const & env, expr const & lhs, expr const & rhs, name_generator const & ngen) {
+    return unify(env, lhs, rhs, ngen, [](constraint const &, name_generator const &) { return lazy_list<constraints>(); });
+}
+
+static int unify_simple(lua_State * L) {
+    int nargs = lua_gettop(L);
+    std::pair<unify_status, substitution> r;
+    if (nargs == 2)
+        r = unify_simple(to_substitution(L, 1), to_constraint(L, 2));
+    else if (nargs == 3 && is_expr(L, 2))
+        r = unify_simple(to_substitution(L, 1), to_expr(L, 2), to_expr(L, 3), justification());
+    else if (nargs == 3 && is_level(L, 2))
+        r = unify_simple(to_substitution(L, 1), to_level(L, 2), to_level(L, 3), justification());
+    else if (is_expr(L, 2))
+        r = unify_simple(to_substitution(L, 1), to_expr(L, 2), to_expr(L, 3), to_justification(L, 4));
+    else
+        r = unify_simple(to_substitution(L, 1), to_level(L, 2), to_level(L, 3), to_justification(L, 4));
+    push_integer(L, static_cast<unsigned>(r.first));
+    push_substitution(L, r.second);
+    return 2;
+}
+
+typedef lazy_list<substitution> substitution_seq;
+DECL_UDATA(substitution_seq)
+
+static const struct luaL_Reg substitution_seq_m[] = {
+    {"__gc", substitution_seq_gc},
+    {0, 0}
+};
+
+static int substitution_seq_next(lua_State * L) {
+    substitution_seq seq = to_substitution_seq(L, lua_upvalueindex(1));
+    substitution_seq::maybe_pair p;
+    p = seq.pull();
+    if (p) {
+        push_substitution_seq(L, p->second);
+        lua_replace(L, lua_upvalueindex(1));
+        push_substitution(L, p->first);
+    } else {
+        lua_pushnil(L);
+    }
+    return 1;
+}
+
+static int push_substitution_seq_it(lua_State * L, substitution_seq const & seq) {
+    push_substitution_seq(L, seq);
+    lua_pushcclosure(L, &safe_function<substitution_seq_next>, 1); // create closure with 1 upvalue
+    return 1;
+}
+
+static void to_constraint_buffer(lua_State * L, int idx, buffer<constraint> & cs) {
+    luaL_checktype(L, idx, LUA_TTABLE);
+    lua_pushvalue(L, idx); // put table on top of the stack
+    int n = objlen(L, idx);
+    for (int i = 1; i <= n; i++) {
+        lua_rawgeti(L, -1, i);
+        cs.push_back(to_constraint(L, -1));
+        lua_pop(L, 1);
+    }
+    lua_pop(L, 1);
+}
+
+static constraints to_constraints(lua_State * L, int idx) {
+    buffer<constraint> cs;
+    to_constraint_buffer(L, idx, cs);
+    return to_list(cs.begin(), cs.end());
+}
+
+static unifier_plugin to_unifier_plugin(lua_State * L, int idx) {
+    luaL_checktype(L, idx, LUA_TFUNCTION); // user-fun
+    luaref f(L, idx);
+    return unifier_plugin([=](constraint const & c, name_generator const & ngen) {
+            lua_State * L = f.get_state();
+            f.push();
+            push_constraint(L, c);
+            push_name_generator(L, ngen);
+            pcall(L, 2, 1, 0);
+            lazy_list<constraints> r;
+            if (is_constraint(L, -1)) {
+                // single constraint
+                r = lazy_list<constraints>(constraints(to_constraint(L, -1)));
+            } else if (lua_istable(L, -1)) {
+                int num = objlen(L, -1);
+                if (num == 0) {
+                    // empty table
+                    r = lazy_list<constraints>();
+                } else {
+                    lua_rawgeti(L, -1, 1);
+                    if (is_constraint(L, -1)) {
+                        // array of constraints case
+                        lua_pop(L, 1);
+                        r = lazy_list<constraints>(to_constraints(L, -1));
+                    } else {
+                        lua_pop(L, 1);
+                        buffer<constraints> css;
+                        // array of array of constraints
+                        for (int i = 1; i <= num; i++) {
+                            lua_rawgeti(L, -1, i);
+                            css.push_back(to_constraints(L, -1));
+                            lua_pop(L, 1);
+                        }
+                        r = to_lazy(to_list(css.begin(), css.end()));
+                    }
+                }
+            } else if (lua_isnil(L, -1)) {
+                // nil case
+                r = lazy_list<constraints>();
+            } else {
+                throw exception("invalid unifier plugin, the result value must be a constrant, "
+                                "nil, an array of constraints, or an array of arrays of constraints");
+            }
+            lua_pop(L, 1);
+            return r;
+        });
+}
+
+static name g_tmp_prefix = name::mk_internal_unique_name();
+
+static int unify(lua_State * L) {
+    int nargs = lua_gettop(L);
+    lazy_list<substitution> r;
+    environment const & env = to_environment(L, 1);
+    if (is_expr(L, 2)) {
+        if (nargs == 3)
+            r = unify(env, to_expr(L, 2), to_expr(L, 3), name_generator(g_tmp_prefix));
+        else if (nargs == 4 && is_name_generator(L, 4))
+            r = unify(env, to_expr(L, 2), to_expr(L, 3), to_name_generator(L, 4));
+        else if (nargs == 4)
+            r = unify(env, to_expr(L, 2), to_expr(L, 3), name_generator(g_tmp_prefix), to_unifier_plugin(L, 4));
+        else
+            r = unify(env, to_expr(L, 2), to_expr(L, 3), to_name_generator(L, 4), to_unifier_plugin(L, 5));
+    } else {
+        buffer<constraint> cs;
+        to_constraint_buffer(L, 2, cs);
+        if (nargs == 2)
+            r = unify(env, cs.size(), cs.data(), name_generator(g_tmp_prefix));
+        else if (nargs == 3 && is_name_generator(L, 3))
+            r = unify(env, cs.size(), cs.data(), to_name_generator(L, 3));
+        else if (nargs == 3)
+            r = unify(env, cs.size(), cs.data(), name_generator(g_tmp_prefix), to_unifier_plugin(L, 3));
+        else
+            r = unify(env, cs.size(), cs.data(), to_name_generator(L, 3), to_unifier_plugin(L, 4));
+    }
+    return push_substitution_seq_it(L, r);
+}
+
+void open_unifier(lua_State * L) {
+    luaL_newmetatable(L, substitution_seq_mt);
+    lua_pushvalue(L, -1);
+    lua_setfield(L, -2, "__index");
+    setfuncs(L, substitution_seq_m, 0);
+    SET_GLOBAL_FUN(substitution_seq_pred, "is_substitution_seq");
+
+    SET_GLOBAL_FUN(unify_simple,  "unify_simple");
+    SET_GLOBAL_FUN(unify,         "unify");
+
+    lua_newtable(L);
+    SET_ENUM("Solved",       unify_status::Solved);
+    SET_ENUM("Failed",       unify_status::Failed);
+    SET_ENUM("Unsupported",  unify_status::Unsupported);
+    lua_setglobal(L, "unify_status");
+}
+}
diff --git a/src/library/unifier.h b/src/library/unifier.h
new file mode 100644
index 000000000..cb9993e21
--- /dev/null
+++ b/src/library/unifier.h
@@ -0,0 +1,44 @@
+/*
+Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#pragma once
+#include <utility>
+#include <functional>
+#include "util/lua.h"
+#include "util/lazy_list.h"
+#include "util/name_generator.h"
+#include "kernel/constraint.h"
+#include "kernel/environment.h"
+#include "kernel/metavar.h"
+
+namespace lean {
+enum class unify_status { Solved, Failed, Unsupported };
+/**
+    \brief Handle the easy-cases: first-order unification, higher-order patterns, identical terms, and terms without metavariables.
+
+    This function assumes that all assigned metavariables have been substituted.
+*/
+std::pair<unify_status, substitution> unify_simple(substitution const & s, expr const & lhs, expr const & rhs, justification const & j);
+std::pair<unify_status, substitution> unify_simple(substitution const & s, level const & lhs, level const & rhs, justification const & j);
+std::pair<unify_status, substitution> unify_simple(substitution const & s, constraint const & c);
+
+/**
+   \brief A unifier_plugin provides a simple way to extend the \c unify procedures.
+   Whenever, the default implementation does not know how to solve a constraint, it invokes the plugin.
+   The plugin return a lazy_list (stream) of possible solutions. Each "solution" is represented as
+   a new list of constraints.
+*/
+typedef std::function<lazy_list<constraints>(constraint const &, name_generator const &)> unifier_plugin;
+
+lazy_list<substitution> unify(environment const & env, unsigned num_cs, constraint const * cs, name_generator const & ngen,
+                              unifier_plugin const & p, bool use_exception = true);
+lazy_list<substitution> unify(environment const & env, unsigned num_cs, constraint const * cs, name_generator const & ngen,
+                              bool use_exception = true);
+lazy_list<substitution> unify(environment const & env, expr const & lhs, expr const & rhs, name_generator const & ngen, unifier_plugin const & p);
+lazy_list<substitution> unify(environment const & env, expr const & lhs, expr const & rhs, name_generator const & ngen);
+
+void open_unifier(lua_State * L);
+}
diff --git a/tests/lua/unify1.lua b/tests/lua/unify1.lua
new file mode 100644
index 000000000..14aa0019b
--- /dev/null
+++ b/tests/lua/unify1.lua
@@ -0,0 +1,55 @@
+function test_unify_simple(lhs, rhs, expected)
+   print(tostring(lhs) .. " =?= " .. tostring(rhs) .. ", expected: " .. tostring(expected))
+   r, s = unify_simple(substitution(), lhs, rhs, justification())
+   if r == unify_status.Solved then
+      s:for_each_expr(function(n, v, j)
+                         print("  " .. tostring(n) .. " := " .. tostring(v))
+      end)
+      s:for_each_level(function(n, v, j)
+                          print("  " .. tostring(n) .. " := " .. tostring(v))
+      end)
+   end
+   assert(r == expected)
+end
+
+local f  = Const("f")
+local a  = Const("a")
+local l1 = mk_local("l1", "x", Bool)
+local l2 = mk_local("l2", "y", Bool)
+local l3 = mk_local("l3", "z", Bool)
+local m  = mk_metavar("m", Bool)
+
+test_unify_simple(m(l1, l2), f(f(a, l1), l1), unify_status.Solved)
+test_unify_simple(m(l1, l2), m(l1, l2), unify_status.Solved)
+test_unify_simple(m(l1, l2), m(l2, l2), unify_status.Unsupported)
+test_unify_simple(m(l1, l2), f(f(l2, l1), l1), unify_status.Solved)
+test_unify_simple(m(l1, a), f(f(a), l1), unify_status.Unsupported)
+test_unify_simple(f(m, a), f(f(a), l1), unify_status.Unsupported)
+test_unify_simple(m(l1, l2), f(f(m), l1), unify_status.Failed)
+test_unify_simple(m(l1, l2), f(f(l3), l1), unify_status.Failed)
+test_unify_simple(m, f(a), unify_status.Solved)
+test_unify_simple(f(a), f(a), unify_status.Solved)
+test_unify_simple(f(a), f(f(a)), unify_status.Failed)
+test_unify_simple(f(f(a, l1), l1), m(l1, l2), unify_status.Solved)
+test_unify_simple(m(l1, l1), f(f(l2, l1), l1), unify_status.Unsupported)
+test_unify_simple(m(l1, l2, l1), f(f(l2, l1), l1), unify_status.Unsupported)
+test_unify_simple(m(l1, l2), Fun(l3, f(f(a, l1), l3)), unify_status.Solved)
+
+local zero = level()
+local one  = zero+1
+local l    = mk_param_univ("l")
+local u    = mk_global_univ("u")
+local m    = mk_meta_univ("m")
+test_unify_simple(m+1, u+1, unify_status.Solved)
+test_unify_simple(u+1, u+1, unify_status.Solved)
+test_unify_simple(u+1, m+1, unify_status.Solved)
+test_unify_simple(m, u+1, unify_status.Solved)
+test_unify_simple(m, max_univ(u, l), unify_status.Solved)
+test_unify_simple(max_univ(u, l), max_univ(u, l), unify_status.Solved)
+test_unify_simple(m+1, m+1, unify_status.Solved)
+test_unify_simple(l, l+1, unify_status.Failed)
+test_unify_simple(m, m+1, unify_status.Failed)
+test_unify_simple(m, max_univ(m, u), unify_status.Unsupported)
+test_unify_simple(m, max_univ(m, u)+1, unify_status.Failed)
+test_unify_simple(m+2, m+1, unify_status.Failed)
+test_unify_simple(u+2, m+1, unify_status.Solved)
diff --git a/tests/lua/unify2.lua b/tests/lua/unify2.lua
new file mode 100644
index 000000000..99bfc625f
--- /dev/null
+++ b/tests/lua/unify2.lua
@@ -0,0 +1,31 @@
+function test_unify(env, lhs, rhs, num_s)
+   print(tostring(lhs) .. " =?= " .. tostring(rhs) .. ", expected: " .. tostring(num_s))
+   local ss = unify(env, lhs, rhs)
+   local n  = 0
+   for s in ss do
+      print("solution: ")
+      s:for_each_expr(function(n, v, j)
+                         print("  " .. tostring(n) .. " := " .. tostring(v))
+      end)
+      s:for_each_level(function(n, v, j)
+                          print("  " .. tostring(n) .. " := " .. tostring(v))
+      end)
+      n = n + 1
+   end
+   assert(num_s == n)
+end
+
+local env = environment()
+env = add_decl(env, mk_var_decl("N", Type))
+local N = Const("N")
+env = add_decl(env, mk_var_decl("f", mk_arrow(N, N, N)))
+env = add_decl(env, mk_var_decl("a", N))
+local f  = Const("f")
+local a  = Const("a")
+local l1 = mk_local("l1", "x", N)
+local l2 = mk_local("l2", "y", N)
+local l3 = mk_local("l3", "z", N)
+local m  = mk_metavar("m", mk_arrow(N, N, N))
+test_unify(env, m(l1, l2), f(f(a, l1), l1), 1)
+test_unify(env, f(m(l1, l2), l1), f(f(a, l1), l1), 1)
+test_unify(env, f(m(l1, l2), a), f(f(a, l1), l1), 0)