let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds (('not' a) 'imp' ('not' b)) 'imp' (b 'imp' a) = I_el Y
let a, b be Function of Y,BOOLEAN; :: thesis: (('not' a) 'imp' ('not' b)) 'imp' (b 'imp' a) = I_el Y
let x be Element of Y; :: according to FUNCT_2:def 8 :: thesis: ((('not' a) 'imp' ('not' b)) 'imp' (b 'imp' a)) . x = (I_el Y) . x

A1: now :: thesis: ( ( a . x = TRUE & ('not' (a . x)) 'or' (a . x) = TRUE ) or ( a . x = FALSE & ('not' (a . x)) 'or' (a . x) = TRUE ) )

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

case a . x = TRUE ; :: thesis: ('not' (a . x)) 'or' (a . x) = TRUE

hence ('not' (a . x)) 'or' (a . x) = TRUE by BINARITH:10; :: thesis: verum

end;

case a . x = FALSE ; :: thesis: ('not' (a . x)) 'or' (a . x) = TRUE

then ('not' (a . x)) 'or' (a . x) = TRUE 'or' FALSE by MARGREL1:11
.= TRUE by BINARITH:10 ;
hence ('not' (a . x)) 'or' (a . x) = TRUE ; :: thesis: verum

end;

end;

end;

A2: now :: thesis: ( ( b . x = TRUE & ('not' (b . x)) 'or' (b . x) = TRUE ) or ( b . x = FALSE & ('not' (b . x)) 'or' (b . x) = TRUE ) )

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

case b . x = TRUE ; :: thesis: ('not' (b . x)) 'or' (b . x) = TRUE

hence ('not' (b . x)) 'or' (b . x) = TRUE by BINARITH:10; :: thesis: verum

end;

case b . x = FALSE ; :: thesis: ('not' (b . x)) 'or' (b . x) = TRUE

then ('not' (b . x)) 'or' (b . x) = TRUE 'or' FALSE by MARGREL1:11
.= TRUE by BINARITH:10 ;
hence ('not' (b . x)) 'or' (b . x) = TRUE ; :: thesis: verum

end;

end;

end;

((('not' a) 'imp' ('not' b)) 'imp' (b 'imp' a)) . x = ('not' ((('not' a) 'imp' ('not' b)) . x)) 'or' ((b 'imp' a) . x) by BVFUNC_1:def 8
.= ('not' (('not' (('not' a) . x)) 'or' (('not' b) . x))) 'or' ((b 'imp' a) . x) by BVFUNC_1:def 8
.= (('not' ('not' (('not' a) . x))) '&' ('not' (('not' b) . x))) 'or' (('not' (b . x)) 'or' (a . x)) by BVFUNC_1:def 8
.= ((('not' (b . x)) 'or' (a . x)) 'or' (('not' a) . x)) '&' ((('not' (b . x)) 'or' (a . x)) 'or' ('not' (('not' b) . x))) by XBOOLEAN:9
.= (('not' (b . x)) 'or' ((a . x) 'or' (('not' a) . x))) '&' ((('not' (b . x)) 'or' (a . x)) 'or' ('not' (('not' b) . x))) by BINARITH:11
.= (('not' (b . x)) 'or' TRUE) '&' ((('not' (b . x)) 'or' (a . x)) 'or' ('not' (('not' b) . x))) by A1, MARGREL1:def 19
.= TRUE '&' ((('not' (b . x)) 'or' (a . x)) 'or' ('not' (('not' b) . x))) by BINARITH:10
.= ((a . x) 'or' ('not' (b . x))) 'or' ('not' (('not' b) . x)) by MARGREL1:14
.= (a . x) 'or' (('not' (b . x)) 'or' ('not' (('not' b) . x))) by BINARITH:11
.= (a . x) 'or' TRUE by A2, MARGREL1:def 19
.= TRUE by BINARITH:10 ;
hence ((('not' a) 'imp' ('not' b)) 'imp' (b 'imp' a)) . x = (I_el Y) . x by BVFUNC_1:def 11; :: thesis: verum