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

A3: now :: thesis: ( ( c . x = TRUE & ('not' (c . x)) 'or' (c . x) = TRUE ) or ( c . x = FALSE & ('not' (c . x)) 'or' (c . x) = TRUE ) )

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

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

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

end;

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

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

end;

end;

end;

((c 'imp' (b 'imp' a)) 'imp' (b 'imp' (c 'imp' a))) . x = ('not' ((c 'imp' (b 'imp' a)) . x)) 'or' ((b 'imp' (c 'imp' a)) . x) by BVFUNC_1:def 8
.= ('not' (('not' (c . x)) 'or' ((b 'imp' a) . x))) 'or' ((b 'imp' (c 'imp' a)) . x) by BVFUNC_1:def 8
.= ('not' (('not' (c . x)) 'or' (('not' (b . x)) 'or' (a . x)))) 'or' ((b 'imp' (c 'imp' a)) . x) by BVFUNC_1:def 8
.= ('not' (('not' (c . x)) 'or' (('not' (b . x)) 'or' (a . x)))) 'or' (('not' (b . x)) 'or' ((c 'imp' a) . x)) by BVFUNC_1:def 8
.= ((c . x) '&' (('not' ('not' (b . x))) '&' ('not' (a . x)))) 'or' (('not' (b . x)) 'or' (('not' (c . x)) 'or' (a . x))) by BVFUNC_1:def 8
.= ((('not' (c . x)) 'or' (a . x)) 'or' ('not' (b . x))) 'or' ((b . x) '&' ((c . x) '&' ('not' (a . x)))) by MARGREL1:16
.= (((('not' (c . x)) 'or' (a . x)) 'or' ('not' (b . x))) 'or' (b . x)) '&' (((('not' (c . x)) 'or' (a . x)) 'or' ('not' (b . x))) 'or' ((c . x) '&' ('not' (a . x)))) by XBOOLEAN:9
.= ((('not' (c . x)) 'or' (a . x)) 'or' TRUE) '&' (((('not' (c . x)) 'or' (a . x)) 'or' ('not' (b . x))) 'or' ((c . x) '&' ('not' (a . x)))) by A1, BINARITH:11
.= TRUE '&' (((('not' (c . x)) 'or' (a . x)) 'or' ('not' (b . x))) 'or' ((c . x) '&' ('not' (a . x)))) by BINARITH:10
.= ((('not' (c . x)) 'or' (a . x)) 'or' ('not' (b . x))) 'or' ((c . x) '&' ('not' (a . x))) by MARGREL1:14
.= (((a . x) 'or' ('not' (b . x))) 'or' ('not' (c . x))) 'or' ((c . x) '&' ('not' (a . x))) by BINARITH:11
.= ((a . x) 'or' ('not' (b . x))) 'or' (('not' (c . x)) 'or' ((c . x) '&' ('not' (a . x)))) by BINARITH:11
.= ((a . x) 'or' ('not' (b . x))) 'or' (TRUE '&' (('not' (c . x)) 'or' ('not' (a . x)))) by A3, XBOOLEAN:9
.= (('not' (b . x)) 'or' (a . x)) 'or' (('not' (c . x)) 'or' ('not' (a . x))) by MARGREL1:14
.= ((('not' (b . x)) 'or' (a . x)) 'or' ('not' (a . x))) 'or' ('not' (c . x)) by BINARITH:11
.= (('not' (b . x)) 'or' TRUE) 'or' ('not' (c . x)) by A2, BINARITH:11
.= TRUE 'or' ('not' (c . x)) by BINARITH:10
.= TRUE by BINARITH:10 ;
hence ((c 'imp' (b 'imp' a)) 'imp' (b 'imp' (c 'imp' a))) . x = (I_el Y) . x by BVFUNC_1:def 11; :: thesis: verum