let Y be non empty set ; :: thesis: for a, b, c being Element of Funcs (Y,BOOLEAN) holds a 'imp' (b 'or' c) = (a '&' ('not' b)) 'imp' c
let a, b, c be Element of Funcs (Y,BOOLEAN); :: thesis: a 'imp' (b 'or' c) = (a '&' ('not' b)) 'imp' c
consider k3 being Function such that
A1: a 'imp' (b 'or' c) = k3 and
A2: dom k3 = Y and
rng k3 c= BOOLEAN by FUNCT_2:def 2;
consider k4 being Function such that
A3: (a '&' ('not' b)) 'imp' c = k4 and
A4: dom k4 = Y and
rng k4 c= BOOLEAN by FUNCT_2:def 2;
for x being Element of Y holds (a 'imp' (b 'or' c)) . x = ((a '&' ('not' b)) 'imp' c) . x

proof

let x be Element of Y; :: thesis: (a 'imp' (b 'or' c)) . x = ((a '&' ('not' b)) 'imp' c) . x
((a '&' ('not' b)) 'imp' c) . x = ('not' ((a '&' ('not' b)) . x)) 'or' (c . x) by BVFUNC_1:def 8
.= (('not' (a . x)) 'or' ('not' (('not' b) . x))) 'or' (c . x) by MARGREL1:def 20
.= (('not' (a . x)) 'or' (b . x)) 'or' (c . x) by MARGREL1:def 19
.= ('not' (a . x)) 'or' ((b . x) 'or' (c . x)) by BINARITH:11
.= ('not' (a . x)) 'or' ((b 'or' c) . x) by BVFUNC_1:def 4
.= (a 'imp' (b 'or' c)) . x by BVFUNC_1:def 8 ;
hence (a 'imp' (b 'or' c)) . x = ((a '&' ('not' b)) 'imp' c) . x ; :: thesis: verum

end;

then for u being set st u in Y holds
k3 . u = k4 . u by A1, A3;
hence a 'imp' (b 'or' c) = (a '&' ('not' b)) 'imp' c by A1, A2, A3, A4, FUNCT_1:2; :: thesis: verum