KeyPathMap.swift

extension Sequence {
    func map<U>(_ keyPath: KeyPath<Element, U>) -> [U] {
        return map { $0[keyPath: keyPath] }
    }

    func flatMap<U>(_ keyPath: KeyPath<Element, U?>) -> [U] {
        return flatMap { $0[keyPath: keyPath] }
    }

    func flatMap<U, S: Sequence>(_ keyPath: KeyPath<Element, S>) -> [U] where S.Iterator.Element == U {
        return flatMap { $0[keyPath: keyPath] }
    }
}