// returns pair(var, function name)
// e.g. for ForWhere(formatFrag("$x"), formatFrag("hasWord($x)"), ...)
static Pair forWhereParseSimpleFunctionCall(ForWhere fw, DeepZipTools tools) {
  if (fw != null && tools.isVar(fw.var) && fw.condition instanceof FunctionCall) {
    FunctionCall fc = cast fw.condition;
    if (eq(fc.args(), ll(fw.var)))
      ret pair(fw.var, fc.function);
  }
  null;
}