let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds (a 'imp' b) 'imp' (a 'imp' a) = I_el Y
let a, b be Function of Y,BOOLEAN; :: thesis: (a 'imp' b) 'imp' (a 'imp' a) = I_el Y
thus (a 'imp' b) 'imp' (a 'imp' a) = (a 'imp' b) 'imp' (I_el Y) by BVFUNC_5:7
.= I_el Y by BVFUNC_6:62 ; :: thesis: verum