Sequence （ Collection ？）を型パラメータにとるのがおもしろいですね。 https://github.com/pointfreeco/swift-nonempty 
A compile-time guarantee that a collection contains a value. - pointfreeco/swift-nonempty
Collection っぽいですね。 https://github.com/pointfreeco/swift-nonempty/blob/master/Sources/NonEmpty/NonEmpty.swift#L1 A compile-time guarantee that a collection contains a value. - pointfreeco/swift-nonempty