[Rd] making a vignette optional

[Therneau, Terry M., Ph.D.]

I had added a vignette to the coxme package and all worked well locally, but it failed at 
CRAN.     The issue is that the vignette involves using coxme for pedigree data, it 
doesn't work without the kinship2 package, and I hadn't put in the necessary "if 
(require(...." logic.

The question is, how do I make the entire vignette conditional? If the package isn't 
available, there is nothing to run.  The latex itself will fail when it can't find the 
figures  (I float them), and the parts that don't will end up as inane discussion of 
material that isn't there.

Terry T.



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