let
p
,
q
be
boolean
object
;
:: thesis:
(
p
<=>
q
=
TRUE
iff (
p
=>
q
=
TRUE
&
q
=>
p
=
TRUE
) )
(
p
=
FALSE
or
p
=
TRUE
)
by
Def3
;
hence
(
p
<=>
q
=
TRUE
iff (
p
=>
q
=
TRUE
&
q
=>
p
=
TRUE
) ) ;
:: thesis:
verum