Variable names in model formula

I just discovered a nice way to do this in S-PLUS the other day; it
doesn't work in R 0.99, but I don't have version 1 installed on this
machine to check there.

The terms() function in S-PLUS returns an expression listing the terms
in a model formula.  It is nice to manipulate:

 formula(terms(somefit)[1]) gives the model formula using just the
first term,

 formula(terms(somefit)[-1]) gives the model formula leaving out the
first term, 

I wrote this for the survival{4,5} packages, where it's used to hack apart
model formulas and meddle with the strata() and cluster() terms.  I think
there's just never been any call for it in the base package before now.

	-thomas

Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

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