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diff --git a/mltt/core/implicits.ML b/mltt/core/implicits.ML
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+(*  Title:      implicits.ML
+    Author:     Joshua Chen
+
+Implicit arguments.
+*)
+
+structure Implicits :
+sig
+
+val implicit_defs: Proof.context -> (term * term) Symtab.table
+val implicit_defs_attr: attribute
+val make_holes: Proof.context -> term list -> term list
+
+end = struct
+
+structure Defs = Generic_Data (
+  type T = (term * term) Symtab.table
+  val empty = Symtab.empty
+  val extend = I
+  val merge = Symtab.merge (Term.aconv o apply2 #1)
+)
+
+val implicit_defs = Defs.get o Context.Proof
+
+val implicit_defs_attr = Thm.declaration_attribute (fn th =>
+  let
+    val (t, def) = Lib.dest_eq (Thm.prop_of th)
+    val (head, args) = Term.strip_comb t
+    val def' = fold_rev lambda args def
+  in
+    Defs.map (Symtab.update (Term.term_name head, (head, def')))
+  end)
+
+fun make_holes_single ctxt tm name_ctxt =
+  let
+    fun iarg_to_hole (Const (\<^const_name>\<open>iarg\<close>, T)) =
+          Const (\<^const_name>\<open>hole\<close>, T)
+      | iarg_to_hole t = t
+
+    fun expand head args =
+      let fun betapplys (head', args') =
+        Term.betapplys (map_aterms iarg_to_hole head', args')
+      in
+        case head of
+          Abs (x, T, t) =>
+            list_comb (Abs (x, T, Lib.traverse_term expand t), args)
+        | _ =>
+            case Symtab.lookup (implicit_defs ctxt) (Term.term_name head) of
+              SOME (t, def) => betapplys
+                (Envir.expand_atom
+                  (Term.fastype_of head)
+                  (Term.fastype_of t, def),
+                args)
+            | NONE => list_comb (head, args)
+      end
+
+    fun holes_to_vars t =
+      let
+        val count = Lib.subterm_count (Const (\<^const_name>\<open>hole\<close>, dummyT))
+
+        fun subst (Const (\<^const_name>\<open>hole\<close>, T)) (Var (idx, _)::_) Ts =
+              let
+                val bounds = map Bound (0 upto (length Ts - 1))
+                val T' = foldr1 (op -->) (Ts @ [T])
+              in
+                foldl1 (op $) (Var (idx, T')::bounds)
+              end
+          | subst (Abs (x, T, t)) vs Ts = Abs (x, T, subst t vs (T::Ts))
+          | subst (t $ u) vs Ts =
+              let val n = count t
+              in subst t (take n vs) Ts $ subst u (drop n vs) Ts end
+          | subst t _ _ = t
+
+        val names = Name.invent name_ctxt "*" (count t)
+        val vars = map (fn n => Var ((n, 0), dummyT)) names
+      in
+        (subst t vars [], fold Name.declare names name_ctxt)
+      end
+  in
+    holes_to_vars (Lib.traverse_term expand tm)
+  end
+
+fun make_holes ctxt tms = #1
+  (fold_map (make_holes_single ctxt) tms (Variable.names_of ctxt))
+
+
+end