Function prop::path_semantics::pand_join

pub fn pand_join<A: Prop, B: Prop, C: Prop, D: Prop>(
    p1: PAndFst<A, B, C, D>, 
    p2: PAndSnd<A, B, C, D>
) -> PAnd<A, B, C, D>

Join PAndFst and PAndSnd.