union function returns a vector set that is the union of the
two sets supplied as arguments. A set is just a row or column vector.
This is not an built-in function. This function
is normally loaded on start-up from the union.r file in the
standard rlib directory. Use of the -r option, incorrectly
setting the RLAB_LIB_DIR environmental variable, or modifying
union.r may make this function unavailable.