This is a command-line utility for doing set operations on files considered as
sets of lines. For instance, `zet union x y z` outputs the lines that occur in
any of `x`, `y`, or `z`.
Two notes:
-Each output line occurs only once, because we're treating the files as sets
 and the lines as their elements.
-We do take the file structure into account in one respect: the lines are
 output in the same order as they are encountered. So `zet union x` prints
 out the lines of `x`, in order, with duplicates removed.