CollectionType.startsWith() -> Index?

extension CollectionType {
    func startsWith<S : SequenceType where S.Generator.Element == Generator.Element>(other: S, @noescape isEquivalent: (Generator.Element, Generator.Element) throws -> Bool) rethrows -> Index? {
        var index = self.startIndex

        // If `self` starts with `other` this will have to iterate over the whole sequence
        for element in other {

            // `self` has to be at least as long as `other`, bail out if not
            if index == self.endIndex {
                return nil
            }

            // Bail out if different
            if try isEquivalent(self[index], element) == false {
                return nil
            }

            // The elements match, continue with the next pair
            index = index.successor()
        }

        return index
    }
}