Zy

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pat:
  | ""
  | paren pat
  | pat \
    | "x"
    | "-"
paren: $pat -> "(" \ pat \ ")"

""
"()"
"x"
"(())"
"()x"
"(x)"
"-"
"((()))"
"()-"
"(()x)"
"xx"
"((x))"
"(())x"
"(-)"
"x-"
"(((())))"
"(())-"
"(()-)"
"()xx"
"((()x))"