let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds ((a 'imp' b) '&' a) 'imp' b = I_el Y
let a, b be Function of Y,BOOLEAN; :: thesis: ((a 'imp' b) '&' a) 'imp' b = I_el Y
((a 'imp' b) '&' a) 'imp' b = (a '&' b) 'imp' b by BVFUNC_6:56;
hence ((a 'imp' b) '&' a) 'imp' b = I_el Y by BVFUNC_6:40; :: thesis: verum