RE: Kevin's excellent question = Is there anyone out there who feels that "before" and
"after" can permit full ellipsis of the predicate as in the following example?

(1) My sister arrived at the movie theater before I [arrived at the theater].

My question, and the one I would ask my students, is, "Why would you want such a construction? For what purpose?" Is anything really 'saved' by not simply finishing the construction with "did"? = My sister arrived at the movie theater before I did.

Just my two cents. Gordon