Revert "change menhir version"

This reverts commit ff33332.

boot/menhir/menhirLib.ml

@@ -1509,8 +1509,7 @@ module Make (T : TABLE) = struct
 
     (* In the legacy strategy, we call [reduce] instead of [announce_reduce],
        apparently in an attempt to hide the reduction steps performed during
-       error handling. This seems inconsistent, as the default reduction steps
-       are still announced. In the simplified strategy, all reductions are
+       error handling. In the simplified strategy, all reductions steps are
        announced. *)
 
     match strategy with
@@ -1546,7 +1545,15 @@ module Make (T : TABLE) = struct
     else begin
 
       (* The stack is nonempty. Pop a cell, updating the current state
-         with that found in the popped cell, and try again. *)
+         to the state [cell.state] found in the popped cell, and continue
+         error handling there. *)
+
+      (* I note that if the new state [cell.state] has a default reduction,
+         then it is ignored. It is unclear whether this is intentional. It
+         could be a good thing, as it avoids a scenario where the parser
+         diverges by repeatedly popping, performing a default reduction of
+         an epsilon production, popping, etc. Still, the question of whether
+         to obey default reductions while error handling seems obscure. *)
 
       let env = { env with
         stack = next;
@@ -3785,5 +3792,5 @@ module MakeEngineTable (T : TableFormat.TABLES) = struct
 end
 end
 module StaticVersion = struct
-let require_20201216 = ()
+let require_20210419 = ()
 end

boot/menhir/menhirLib.mli

@@ -1803,5 +1803,5 @@ module MakeEngineTable
      and type nonterminal = int
 end
 module StaticVersion : sig
-val require_20201216: unit
+val require_20210419: unit
 end