svoid tok_onCleanExit(LS tok) {
  int i;
  while ((i = jfind(tok, "on clean exit {")) >= 0) {
    int iCurly = indexOf(tok, i, "{");
    int iCurlyEnd = findEndOfBracketPart(tok, iCurly)-1;
    int iRest = iCurlyEnd+2;
    int iEndOfOuterBlock = findEndOfBracketPart(tok, iCurlyEnd)-1;
    
    S code = joinSubList(tok, iCurly, iCurlyEnd+1);
    S var = makeVar();
    
    tokPrepend_reTok(tok, iEndOfOuterBlock, "} on fail { set \*var*/; } finally { if (!\*var*/) \*code*/ } } ");
    tokReplace_reTok(tok, i, iRest, "{ bool \*var*/; try { ");
  }
}