children property
Returns a list of directly referenced parsers.
For example, letter().children returns the empty collection [],
because the letter parser is a primitive or leaf parser that does not
depend or call any other parser.
In contrast, letter().or(digit()).children returns a collection
containing both the letter() and digit() parser.
Override this method and Parser.replace in all subclasses that reference other parsers.
Implementation
@override
List<Parser> get children => [
  parser1,
  parser2,
  parser3,
  parser4,
  parser5,
  parser6,
  parser7,
  parser8,
  parser9,
];