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