Just a language nitpick:
Set([7,8,9]).followed(by: [1,2,3])
This doesn't make much sense, semantically. Sets are not sequences; the union of A and B is not accurately described by "A followed by B".
Just a language nitpick:
Set([7,8,9]).followed(by: [1,2,3])
This doesn't make much sense, semantically. Sets are not sequences; the union of A and B is not accurately described by "A followed by B".