diff5.swift

public func diff4<T1, T2>(_ first: [T1], _ second: [T2], with compare: (T1,T2) -> Bool) -> SequenceDiff<T1, T2> {
    let combinations = first.flatMap { firstElement in (firstElement, second.first { secondElement in compare(firstElement, secondElement) }) }
    let common = combinations.filter { $0.1 != nil }.flatMap { ($0.0, $0.1!) }
    let removed = combinations.filter { $0.1 == nil }.flatMap { ($0.0) }
    let inserted = second.filter { secondElement in !common.contains { compare($0.0, secondElement) } }
    
    return SequenceDiff(common: common, removed: removed, inserted: inserted)
}