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

y
)

(
y

(
x

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

y
)

(
y

(
x

z
)
)
=
y
z

x
=
x

z
by
Th20
;
hence
(
x

y
)

(
y

(
x

z
)
)
=
y
by
Th22
;
:: thesis:
verum