let Y be non empty set ; :: thesis: for a, b being Element of Funcs Y,BOOLEAN holds a 'or' (a 'imp' b) = I_el Y
let a, b be Element of Funcs Y,BOOLEAN ; :: thesis: a 'or' (a 'imp' b) = I_el Y
a 'or' (a 'imp' b) = a 'or' (('not' a) 'or' b) by BVFUNC_4:8
.= (a 'or' ('not' a)) 'or' b by BVFUNC_1:11
.= (I_el Y) 'or' b by BVFUNC_4:6 ;
hence a 'or' (a 'imp' b) = I_el Y by BVFUNC_1:13; :: thesis: verum