Quadratic.swift

struct Quadratic<A> {
    let x²: A
    let x: A
    let c: A
}

extension Quadratic: Equatable where A: Equatable {}

extension Linear: FancyMult where A: Additive, A: Multiplicative {
    typealias FancyVal = Quadratic<A>
    static func ** (lhs: Linear<A>, rhs: Linear<A>) -> Quadratic<A> {
        return Quadratic(x²: lhs.x * rhs.x,
                         x: lhs.x * rhs.c + lhs.c * rhs.x,
                         c: lhs.c * rhs.c)
    }
}