copy method
Returns a shallow copy of the receiver.
Override this method in all subclasses, return its own type.
Implementation
@override
SequenceParser9<R1, R2, R3, R4, R5, R6, R7, R8, R9> copy() =>
SequenceParser9<R1, R2, R3, R4, R5, R6, R7, R8, R9>(parser1, parser2,
parser3, parser4, parser5, parser6, parser7, parser8, parser9);