Fetch items from a Swift collection using negative indices

get.swift

import Swift

extension CollectionType where Index: SignedNumberType, Index == Index.Distance {
    func get(var position: Self.Index) -> Self.Generator.Element? {
        if position < 0 {
            position = self.endIndex.advancedBy(position)
        }
        return (indices ~= position) ? self[position] : nil

    }
}

let a = [1, 2, 3, 4, 5]

a.get(-2) // returns 4