[R] terms() and I( ) -- extract variables names only

[Álvaro A. Novo]

Hi,

I am having difficulties extracting all variables used in a formula. Here's 
an example of what I am doing:

> my.formula <- v1 ~ v2 + v3 + I(v4/v5)
> out.terms <- terms(my.formula)
> paste(attr(out.terms, "variables"))[-1]
[1] "v1"       "v2"       "v3"       "I(v4/v5)"

BUT what I would like to get is

[1] "v1"       "v2"       "v3"       "v4"       "v5"

Also, if I change I(v4/v5) to I(v4^2) or any other expression, it should only 
extract the variable (here, V4).

Is there a built-in function to do it? Suggestions are extremely appreciated. 
Thank you for your attention,

Alvaro A. Novo
R-1.2.0
Linux SuSE 6.4
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