Use of transform()

Hi,

Your package was mentioned in the R bugzilla today because of how you use `transform()`.
https://bugs.r-project.org/bugzilla/show_bug.cgi?id=17890  (comment 3).

As you can see from the discussion, `transform()` is not recommended for progamming and `within()` is mentioned as the suggested alternative. 

I just wanted to make you aware of that -- I think it would be an easy fix.