[Rd] question on latticeParseFormula (PR#2602)

[rmh@surfer.sbm.temple.edu]

I didn't get a warning.  I got exactly what I sent.

The NA doesn't bother me since I am interested in parsing the expression,
specifically to see what is to the left of the "~" and what is to the
right of the "~".   I am interested in components left.name and right.name,
.I am not interested in left and right.  My inference from your response
is that I should work with the parse tree from 
  parse(text="y ~ a + b")
rather than continue pushing latticeParseFormula.

On the other issue that you raised, I am surprised to learn that R
can add factors to factors and ordered to ordered.  I didn't expect either
case.  However, given that it can do something, I still would expect
that it could add them to each other by recognizing that ordered inherits
from factor.

> ordered(1:3) + ordered(3:5)
[1] NA NA NA
> factor(1:3) + factor(3:5)
[1] NA NA NA
> factor(1:3) + ordered(3:5)
Error in factor(1:3) + ordered(3:5) : non-numeric argument to binary operator
>

thank you for the prompt response.
Rich