let Y be non empty set ; :: thesis: for a, b being Element of Funcs Y,BOOLEAN holds (a 'imp' b) '&' (a 'imp' ('not' b)) = 'not' a
let a, b be Element of Funcs Y,BOOLEAN ; :: thesis: (a 'imp' b) '&' (a 'imp' ('not' b)) = 'not' a
A1: for x being Element of Y holds ((a 'imp' b) '&' (a 'imp' ('not' b))) . x = ('not' a) . x

proof

let x be Element of Y; :: thesis: ((a 'imp' b) '&' (a 'imp' ('not' b))) . x = ('not' a) . x
A2: ((a 'imp' b) '&' (a 'imp' ('not' b))) . x = ((a 'imp' b) . x) '&' ((a 'imp' ('not' b)) . x) by MARGREL1:def 21
.= (('not' (a . x)) 'or' (b . x)) '&' ((a 'imp' ('not' b)) . x) by BVFUNC_1:def 11
.= (('not' (a . x)) 'or' (b . x)) '&' (('not' (a . x)) 'or' (('not' b) . x)) by BVFUNC_1:def 11
.= (('not' (a . x)) 'or' (b . x)) '&' (('not' (a . x)) 'or' ('not' (b . x))) by MARGREL1:def 20 ;

now

per cases ( b . x = TRUE or b . x = FALSE ) by XBOOLEAN:def 3;

case b . x = TRUE ; :: thesis: ((a 'imp' b) '&' (a 'imp' ('not' b))) . x = ('not' a) . x

then ((a 'imp' b) '&' (a 'imp' ('not' b))) . x = (('not' (a . x)) 'or' TRUE ) '&' (('not' (a . x)) 'or' FALSE ) by A2, MARGREL1:41
.= (('not' (a . x)) 'or' TRUE ) '&' ('not' (a . x)) by BINARITH:7
.= TRUE '&' ('not' (a . x)) by BINARITH:19
.= 'not' (a . x) by MARGREL1:50
.= ('not' a) . x by MARGREL1:def 20 ;
hence ((a 'imp' b) '&' (a 'imp' ('not' b))) . x = ('not' a) . x ; :: thesis: verum

end;

case b . x = FALSE ; :: thesis: ((a 'imp' b) '&' (a 'imp' ('not' b))) . x = ('not' a) . x

then ((a 'imp' b) '&' (a 'imp' ('not' b))) . x = (('not' (a . x)) 'or' FALSE ) '&' (('not' (a . x)) 'or' TRUE ) by A2, MARGREL1:41
.= ('not' (a . x)) '&' (('not' (a . x)) 'or' TRUE ) by BINARITH:7
.= TRUE '&' ('not' (a . x)) by BINARITH:19
.= 'not' (a . x) by MARGREL1:50
.= ('not' a) . x by MARGREL1:def 20 ;
hence ((a 'imp' b) '&' (a 'imp' ('not' b))) . x = ('not' a) . x ; :: thesis: verum

end;

end;

end;

hence ((a 'imp' b) '&' (a 'imp' ('not' b))) . x = ('not' a) . x ; :: thesis: verum

end;

consider k3 being Function such that
A3: ( (a 'imp' b) '&' (a 'imp' ('not' b)) = k3 & dom k3 = Y & rng k3 c= BOOLEAN ) by FUNCT_2:def 2;
consider k4 being Function such that
A4: ( 'not' a = k4 & dom k4 = Y & rng k4 c= BOOLEAN ) by FUNCT_2:def 2;
( Y = dom k3 & Y = dom k4 & ( for u being set st u in Y holds
k3 . u = k4 . u ) ) by A1, A3, A4;
hence (a 'imp' b) '&' (a 'imp' ('not' b)) = 'not' a by A3, A4, FUNCT_1:9; :: thesis: verum