let Y be non empty set ; :: thesis: for a, b, c being Function of Y,BOOLEAN holds (a 'imp' b) '&' (b 'imp' c) = ((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)
let a, b, c be Function of Y,BOOLEAN; :: thesis: (a 'imp' b) '&' (b 'imp' c) = ((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)
let x be Element of Y; :: according to FUNCT_2:def 8 :: thesis: ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x
A5: (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) . x) '&' ((a 'imp' c) . x) by MARGREL1:def 20
.= (((a 'imp' b) . x) '&' ((b 'imp' c) . x)) '&' ((a 'imp' c) . x) by MARGREL1:def 20
.= ((('not' (a . x)) 'or' (b . x)) '&' ((b 'imp' c) . x)) '&' ((a 'imp' c) . x) by BVFUNC_1:def 8
.= ((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' ((a 'imp' c) . x) by BVFUNC_1:def 8
.= ((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' (('not' (a . x)) 'or' (c . x)) by BVFUNC_1:def 8
.= (((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' ('not' (a . x))) 'or' (((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' (c . x)) by XBOOLEAN:8 ;
A6: (('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x)) = ((a 'imp' b) . x) '&' (('not' (b . x)) 'or' (c . x)) by BVFUNC_1:def 8
.= ((a 'imp' b) . x) '&' ((b 'imp' c) . x) by BVFUNC_1:def 8
.= ((a 'imp' b) '&' (b 'imp' c)) . x by MARGREL1:def 20 ;
A7: ((a 'imp' b) '&' (b 'imp' c)) . x = ((a 'imp' b) . x) '&' ((b 'imp' c) . x) by MARGREL1:def 20
.= (('not' (a . x)) 'or' (b . x)) '&' ((b 'imp' c) . x) by BVFUNC_1:def 8
.= (('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x)) by BVFUNC_1:def 8 ;

now :: thesis: ( ( a . x = TRUE & c . x = TRUE & ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x ) or ( a . x = TRUE & c . x = FALSE & ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x ) or ( a . x = FALSE & c . x = TRUE & ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x ) or ( a . x = FALSE & c . x = FALSE & ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x ) )

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

case ( a . x = TRUE & c . x = TRUE ) ; :: thesis: ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x

then (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x = (((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' FALSE) 'or' (((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' TRUE) by A5, MARGREL1:11
.= FALSE 'or' (((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' TRUE) by MARGREL1:13
.= FALSE 'or' ((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) by MARGREL1:14
.= ((a 'imp' b) '&' (b 'imp' c)) . x by A6, BINARITH:3 ;
hence ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x ; :: thesis: verum

end;

case A8: ( a . x = TRUE & c . x = FALSE ) ; :: thesis: ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x

then A9: ((a 'imp' b) '&' (b 'imp' c)) . x = (FALSE 'or' (b . x)) '&' (('not' (b . x)) 'or' FALSE) by A7, MARGREL1:11
.= (FALSE 'or' (b . x)) '&' ('not' (b . x)) by BINARITH:3
.= (b . x) '&' ('not' (b . x)) by BINARITH:3
.= FALSE by XBOOLEAN:138 ;
(((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x = (((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' FALSE) 'or' (((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' FALSE) by A5, A8, MARGREL1:11
.= FALSE by MARGREL1:13 ;
hence ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x by A9; :: thesis: verum

end;

case ( a . x = FALSE & c . x = TRUE ) ; :: thesis: ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x

then (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x = (((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' TRUE) 'or' (((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) '&' TRUE) by A5, MARGREL1:11
.= ((a 'imp' b) '&' (b 'imp' c)) . x by A6, MARGREL1:14 ;
hence ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x ; :: thesis: verum

end;

case ( a . x = FALSE & c . x = FALSE ) ; :: thesis: ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x

then (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x = (TRUE '&' ((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x)))) 'or' (FALSE '&' ((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x)))) by A5, MARGREL1:11
.= ((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) 'or' (FALSE '&' ((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x)))) by MARGREL1:14
.= ((('not' (a . x)) 'or' (b . x)) '&' (('not' (b . x)) 'or' (c . x))) 'or' FALSE by MARGREL1:13
.= ((a 'imp' b) '&' (b 'imp' c)) . x by A6, BINARITH:3 ;
hence ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x ; :: thesis: verum

end;

end;

end;

hence ((a 'imp' b) '&' (b 'imp' c)) . x = (((a 'imp' b) '&' (b 'imp' c)) '&' (a 'imp' c)) . x ; :: thesis: verum