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