pub fn in_left_arg<A: Prop, B: Prop, C: Prop>( Sq: Sq<A, B>, (_, g1): Eq<A, C> ) -> Sq<C, B>
(a ¬> b) ∧ (a == c) => (c ¬> b).
(a ¬> b) ∧ (a == c) => (c ¬> b)