let
X
,
Y
be
set
;
:: thesis:
X
\+\
Y
=
(
X
\/
Y
)
\
(
X
/\
Y
)
thus
X
\+\
Y
=
(
X
\
(
X
/\
Y
)
)
\/
(
Y
\
X
)
by
Th47
.=
(
X
\
(
X
/\
Y
)
)
\/
(
Y
\
(
X
/\
Y
)
)
by
Th47
.=
(
X
\/
Y
)
\
(
X
/\
Y
)
by
Th42
;
:: thesis:
verum