pub fn inv_triangle<A: DProp, B: DProp, C: DProp>( f: Eq<Not<Eq<A, B>>, Not<Eq<C, B>>>) -> Eq<A, C>
¬(a = b) = ¬(c = b) => (a = c).
¬(a = b) = ¬(c = b) => (a = c)