copy method
Returns a shallow copy of the receiver.
Override this method in all subclasses, return its own type.
Implementation
@override
SequenceParser6<R1, R2, R3, R4, R5, R6> copy() =>
SequenceParser6<R1, R2, R3, R4, R5, R6>(
parser1, parser2, parser3, parser4, parser5, parser6);