Function prop::eq::in_left

pub fn in_left<A: Prop, B: Prop, C: Prop, F, G>(
    eq_ab: Eq<A, B>,
    f: F,
    g: G
) -> Eq<C, B>where
    F: Fn(A) -> C + 'static,
    G: Fn(C) -> A + 'static,

Makes it easier to traverse.