exit status: 1 with stderr:
<stdin>:10:7: error: ambiguous reference to member 'flatten()' print(b.flatten()) ^ Swift.Sequence:27:17: note: found this candidate public func flatten() -> FlattenSequence<Self> ^ <stdin>:2:17: note: found this candidate public func flatten<T>() -> [T] where Element : Sequence, Element.Element == T { ^ Swift.Collection:27:17: note: found this candidate public func flatten() -> FlattenCollection<Self> ^ Swift.BidirectionalCollection:27:17: note: found this candidate public func flatten() -> FlattenBidirectionalCollection<Self> ^