consider
A1
,
A2
being
Subset
of
U
such that
A1
: (
A1
c=
A2
&
A
=
Inter
(
A1
,
A2
) )
by
Th11
;
thus
ex
b
_{1}
,
B
being
Subset
of
U
st
A
=
Inter
(
b
_{1}
,
B
)
by
A1
;
:: thesis:
verum