let Y be non empty set ; :: thesis: (O_el Y) 'eqv' (O_el Y) = I_el Y
thus (O_el Y) 'eqv' (O_el Y) = 'not' ((O_el Y) 'xor' (O_el Y)) by Th12
.= 'not' (O_el Y) by Th13
.= I_el Y by BVFUNC_1:2 ; :: thesis: verum