let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds a 'eqv' b = 'not' (a 'xor' b)
let a, b be Function of Y,BOOLEAN; :: thesis: a 'eqv' b = 'not' (a 'xor' b)
'not' (a 'xor' b) = 'not' ('not' (a 'eqv' b)) by BVFUNC_6:85;
hence a 'eqv' b = 'not' (a 'xor' b) ; :: thesis: verum