Revert "Lexer: Forbid producer identifier of the form _[0-9]+."

This reverts commit fc203a594b2d68f40b18966550e9c23460fc20e9.

src/lexer.mll

@@ -312,11 +312,6 @@ rule main = parse
     { STAR }
 | "+"
     { PLUS }
-| "_" ['0'-'9']+
-    { Error.errorp
-        (Positions.with_poss (lexeme_start_p lexbuf) (lexeme_end_p lexbuf) ())
-        "identifiers of the form _0, _1, _2, ..., are reserved."
-    }
 | (lowercase identchar *) as id
     { if Hashtbl.mem reserved id then
 	Error.errorp