My dictionary relates 'tautology' to needless repetition. 

That's another form of the x.c' <> x.c-vt idiocy. 

The repetition involved is the vt transformation term.
Apply the -vt term to the x term, and it is needless 
repetition to apply it anywhere again? The 'again' is
to the x.c term.  The x.c' = x.c crackpot idiocy.

The repetition of the vt terms is required by the presence
of two x values to be transformed.

Be sure to note the next section.