SafeRemoveIndex.swift

public extension RangeReplaceableCollectionType where Index: Comparable {
    public subscript (safe index: Index) -> Generator.Element? {
        get {
            guard indices ~= index else { return nil }
            return self[index]
        }
        set {
            guard indices ~= index else { return }
            if let newValue = newValue {
                self.removeAtIndex(index)
                self.insert(newValue, atIndex: index)
            }
        }
    }
    
    public mutating func removeAtIndex(safe index: Self.Index) -> Self.Generator.Element? {
        guard indices ~= index else { return nil}
        return self.removeAtIndex(index)
    }
}