let Y be non empty set ; :: thesis: for a, b, c being Function of Y,BOOLEAN holds a 'nor' (b 'or' c) = 'not' ((a 'or' b) 'or' c)
let a, b, c be Function of Y,BOOLEAN; :: thesis: a 'nor' (b 'or' c) = 'not' ((a 'or' b) 'or' c)
thus a 'nor' (b 'or' c) = 'not' (a 'or' (b 'or' c)) by Th2
.= 'not' ((a 'or' b) 'or' c) by BVFUNC_1:8 ; :: thesis: verum