[Graphics:submitted.nbgr326.gif] [Graphics:submitted.nbgr327.gif].

For this, we take in [Graphics:submitted.nbgr328.gif] all variables arbitrary but fixed and prove

[Graphics:submitted.nbgr329.gif] [Graphics:submitted.nbgr330.gif]

A proof by simplification works.

Simplification of the lhs term:

[Graphics:submitted.nbgr331.gif] = by [Graphics:submitted.nbgr332.gif]

[Graphics:submitted.nbgr333.gif]

Simplification of the rhs term:

[Graphics:submitted.nbgr334.gif]