Address #240: Speed up union by using a hashtable
Use a hashtable to speed up union
when the test is suitable
and the lists are long enough. Basically like the changes for set-difference
where we hash the second list if it's long enough.
Use a hashtable to speed up union
when the test is suitable
and the lists are long enough. Basically like the changes for set-difference
where we hash the second list if it's long enough.