build method
Implementation
@override
String build(Context context, ParserResult? result) {
  final fast = result == null;
  final values = context.allocateLocals(['pos']);
  values.addAll({
    'p1': before.build(context, null),
    'p2': parser.build(context, result),
    'p3': after.build(context, null),
  });
  return render2(fast, _templateFast, _template, values, [result]);
}