parse method
Parses the input and returns a result wrapped in Result if successful,
null
otherwise.
Implementation
@override
Result<({int start, int end, String value})>? parse(
State<StringReader> state) {
final pos = state.pos;
final r = p.parse(state);
if (r != null) {
final v = state.input.substring(pos, state.pos);
return Result((start: pos, end: state.pos, value: v));
}
return null;
}