let Y be non empty set ; :: thesis: (O_el Y) 'eqv' (I_el Y) = O_el Y
thus (O_el Y) 'eqv' (I_el Y) = 'not' ((O_el Y) 'xor' (I_el Y)) by Th12
.= 'not' ((O_el Y) 'xor' ('not' (O_el Y))) by BVFUNC_1:2
.= 'not' (I_el Y) by Th14
.= O_el Y by BVFUNC_1:2 ; :: thesis: verum