let
L
be non
empty
satisfying_Sh_1
ShefferStr
;
:: thesis:
for
x
,
y
being
Element
of
L
holds
(
(
x

y
)

(
x

y
)
)

y
=
x

y
let
x
,
y
be
Element
of
L
;
:: thesis:
(
(
x

y
)

(
x

y
)
)

y
=
x

y
(
y

y
)

(
x

y
)
=
y
by
Th11
;
hence
(
(
x

y
)

(
x

y
)
)

y
=
x

y
by
Th11
;
:: thesis:
verum