I'm writing my first S3 class and associated methods and I would like to know how to subset my input data set in order to keep only the variables specified in the formula?
f <- Species~Petal.Length + Petal.Width
labels(terms(f)) #  "Petal.Length" "Petal.Width"
labels.terms returns the
"term.labels" attribute of a
terms object, which excludes the LHS variable.