let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds a 'imp' (a 'nor' b) = 'not' (a 'or' (a '&' b))
let a, b be Function of Y,BOOLEAN; :: thesis: a 'imp' (a 'nor' b) = 'not' (a 'or' (a '&' b))
thus a 'imp' (a 'nor' b) = 'not' (a '&' (a 'or' b)) by Th65
.= 'not' ((a '&' a) 'or' (a '&' b)) by BVFUNC_1:12
.= 'not' (a 'or' (a '&' b)) ; :: thesis: verum