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

y
=
y

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

y
=
y

x
x

(
(
y

x
)

x
)
=
y

x
by
Th14
;
hence
x

y
=
y

x
by
Th19
;
:: thesis:
verum