Function prop::eq::transpose

pub fn transpose<A: DProp, B: DProp, C: DProp, D: DProp>(
    f: Eq<Eq<A, B>, Eq<C, D>>
) -> Eq<Eq<A, C>, Eq<B, D>>

(a = b) = (c = d) => (a = c) = (b = d).