let Y be non empty set ; :: thesis: for a, b, c being Element of Funcs Y,BOOLEAN holds a 'nor' (b 'eqv' c) = ('not' a) '&' (b 'xor' c)
let a, b, c be Element of Funcs Y,BOOLEAN ; :: thesis: a 'nor' (b 'eqv' c) = ('not' a) '&' (b 'xor' c)
thus a 'nor' (b 'eqv' c) = 'not' (a 'or' (b 'eqv' c)) by Th2
.= ('not' a) '&' ('not' (b 'eqv' c)) by BVFUNC_1:16
.= ('not' a) '&' ('not' ('not' (b 'xor' c))) by BVFUNC25:12
.= ('not' a) '&' (b 'xor' c) ; :: thesis: verum