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

(
y

z
)
)

(
y

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

(
y

z
)
)

(
y

x
)
=
x
z

y
=
y

z
by
Th20
;
hence
(
x

(
y

z
)
)

(
y

x
)
=
x
by
Th23
;
:: thesis:
verum