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

y
)

(
x

z
)
)

z
=
x

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

y
)

(
x

z
)
)

z
=
x

z
(
x

z
)

(
z

(
x

y
)
)
=
z
by
Th24
;
hence
(
(
x

y
)

(
x

z
)
)

z
=
x

z
by
Th22
;
:: thesis:
verum