set
A
=
{
x
where
x
is
Subset
of
T
: (
x
is
open
&
x
is
closed
)
}
;
{}
T
in
{
x
where
x
is
Subset
of
T
: (
x
is
open
&
x
is
closed
)
}
;
hence
not
OpenClosedSet
T
is
empty
;
:: thesis:
verum