denumerate converts a formula written using the conventions of
loglm into one that terms is able to process. renumerate
converts it back again to a form like the original.
A formula where all variables with names of the form .vn, where
n is an integer, converted to numbers, n, as allowed by the
formula conventions of loglm.
Details
This is an inverse function to denumerate. It is only needed
since terms returns an expanded form of the original formula
where the non-marginal terms are exposed. This expanded form is
mapped back into a form corresponding to the one that the user
originally supplied.