// void blu(optional S bla) { ... }
// => void blu(S bla default null) { ... }
svoid tok_optionalArguments(LS tok) {
  for (int i : jfindAll_reverse(tok, "optional <id>", (_tok, nIdx)
    -> eqGetOneOf(tok, nIdx-1, ",", "("))) {
    int iEndOfType = tok_endOfType(tok, i+2);
    S var = tok.get(iEndOfType);
    S type = joinSubList(tok, i+2, iEndOfType);
    
    tokReplace_reTok(tok, i, iEndOfType+1,
      "\*type*/ \*var*/ default null");
  }
}

Began life as a copy of #1031988