let Y be non empty set ; :: thesis: for a, b being Function of Y,BOOLEAN holds b 'imp' ((b 'imp' a) 'imp' a) = I_el Y
let a, b be Function of Y,BOOLEAN; :: thesis: b 'imp' ((b 'imp' a) 'imp' a) = I_el Y
let x be Element of Y; :: according to FUNCT_2:def 8 :: thesis: (b 'imp' ((b 'imp' a) 'imp' a)) . x = (I_el Y) . x

A1: 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;

A2: 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;

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