[R] Using xlevels

[Terry Therneau]

I'm working on predict.survreg and am confused about xlevels.
The model.frame method has the argument, but none of the standard
methods (model.frame.lm, model.frame.glm) appear to make use of it.

The documentation for model.matrix states:
  xlev: to be used as argument of model.frame if data has no "terms"
attribute.

But the terms attribute has no xlevels information in it, so I find this
statement completely confusing.  Any insight is appreciated.

Terry Therneau