public prefix func - (v: CGVector) -> CGVector {
    return CGVector(dx: -v.dx, dy: -v.dy)
}

public func + (p: CGPoint, v: CGVector) -> CGPoint {
    return CGPoint(x: p.x + v.dx, y: p.y + v.dy)
}

public func - (p: CGPoint, v: CGVector) -> CGPoint {
    return p + (-v)
}

public func - (lhs: CGPoint, rhs: CGPoint) -> CGVector {
    return CGVector(dx: rhs.x - lhs.x, dy: rhs.y - lhs.y)
}


let points = [CGPoint.zero, CGPoint.zero, CGPoint.zero]
let startPoint = points[0] - (points[1] - points[0]) // ambiguous reference to member '-'

error: ambiguous reference to member '-'
let startPoint = points[0] - (points[1] - points[0])
                                        ^

note: found this candidate
public func - (p: CGPoint, v: CGVector) -> CGPoint {
            ^

note: found this candidate
public func - (lhs: CGPoint, rhs: CGPoint) -> CGVector {
            ^