reconsider
A
=
C
,
B
=
D
as
Subset
of
T
;
A5
:
A
is
open
by
Th1
;
A6
:
B
is
open
by
Th1
;
A7
:
A
is
closed
by
Th2
;
A8
:
B
is
closed
by
Th2
;
A
/\
B
is
open
by
A5
,
A6
;
hence
C
/\
D
is
Element
of
OpenClosedSet
T
by
A7
,
A8
,
Th3
;
:: thesis:
verum