joined(by:)

Returns the concatenation of the elements in this sequence of sequences, inserting the given separator between each sequence.

Joined.swift:335
func joined(by separator: Element.Element) -> JoinedBySequence<Self, CollectionOfOne<Element.Element>>
for x in [[1, 2], [3, 4], [5, 6]].joined(by: 100) {
    print(x)
}
// 1, 2, 100, 3, 4, 100, 5, 6