[Rd] backquotes and term.labels

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

A user reported a problem with the survdiff function and the use of variables that contain 
a space.  Here is a simple example.  The same issue occurs in survfit for the same reason.

lung2 <- lung
names(lung2)[1] <- "in st"   # old name is inst
survdiff(Surv(time, status) ~ `in st`, data=lung2)
Error in `[.data.frame`(m, ll) : undefined columns selected

In the body of the code the program want to send all of the right-hand side variables 
forward to the strata() function.  The code looks more or less like this, where m is the 
model frame

   Terms <- terms(m)
   index <- attr(Terms, "term.labels")
   if (length(index) ==0)  X <- rep(1L, n)  # no coariates
   else X <- strata(m[index])

For the variable with a space in the name the term.label is "`in st`", and the subscript 
fails.

Is this intended behaviour or a bug?  The issue is that the name of this column in the 
model frame does not have the backtics, while the terms structure does have them.

Terry T.