theory Xxxxx imports Pure

keywords "yyyyy" :: prf_open
begin

ML \<open>
  @{keyword "("} : string parser; (* this works as expected *)

  val _ =
    Outer_Syntax.command @{command_keyword yyyyy} "dummy definition yyyyy"
      (Scan.succeed (Toplevel.proof Proof.begin_block));

  @{keyword "yyyyy"};             (*ERROR Bad outer syntax keyword "yyyyy"*)
\<close>
end