search for: expr12

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