Dear R fellows,
 
Assume I define

a <- expression(fn+tp)
sen <- expression(tp/a)

Now I'd like to know, which variables are necessary for calculating sen

all.vars(sen)

This results in a vector c(tp,a). But I'd like all.vars to evaluate the
sen-object down to the ground level, which would result in a vector
c(tp,fn) (because a was defined as fn+tp). In other words, I'd like
all.vars to expand the a-object (and all other downstream objects). I am
looking for a solution, that works with much more levels. This is just a
very simple example.

I'd appreciate any suggestions how to do that very much!
Thanks in advance,

Felix

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