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

y
)

(
x

(
z

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

y
)

(
x

(
z

y
)
)
=
x
y

(
(
x

y
)

y
)
=
x

y
by
Th14
;
hence
(
x

y
)

(
x

(
z

y
)
)
=
x
by
Def1
;
:: thesis:
verum