Add missing source file

(I did add this, then I messed around with git and apparently broke it
without noticing. Oops!)

src/TTImp/Elab/Utils.idr

@@ -0,0 +1,40 @@
+module TTImp.Elab.Utils
+
+import Core.Context
+import Core.Core
+import Core.Env
+import Core.Normalise
+import Core.TT
+import Core.Value
+
+import TTImp.TTImp
+
+findErasedFrom : Defs -> Nat -> NF [] -> Core (List Nat)
+findErasedFrom defs pos (NBind fc x (Pi c _ _) scf)
+    = do sc <- scf defs (toClosure defaultOpts [] (Erased fc))
+         rest <- findErasedFrom defs (1 + pos) sc
+         case c of
+              Rig0 => pure (pos :: rest)
+              _ => pure rest
+findErasedFrom defs pos tm = pure []
+
+-- Find the argument positions in the given type which are guaranteed to be
+-- erasable
+export
+findErased : {auto c : Ref Ctxt Defs} ->
+             ClosedTerm -> Core (List Nat)
+findErased tm 
+    = do defs <- get Ctxt
+         tmnf <- nf defs [] tm
+         findErasedFrom defs 0 tmnf
+
+export
+updateErasable : {auto c : Ref Ctxt Defs} ->
+                 Name -> Core ()
+updateErasable n
+    = do defs <- get Ctxt
+         Just gdef <- lookupCtxtExact n (gamma defs)
+              | Nothing => pure ()
+         es <- findErased (type gdef)
+         addDef n (record { eraseArgs = es } gdef)
+         pure ()