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

(
(
y

x
)

x
)
=
y

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

(
(
y

x
)

x
)
=
y

x
(
(
y

x
)

(
y

x
)
)

x
=
y

x
by
Th13
;
hence
x

(
(
y

x
)

x
)
=
y

x
by
Th10
;
:: thesis:
verum