Union matches: select type by number of matching fields

[m-aciek]

Closes #263.

[airunderscoreland]

This looks like it safely solves the issue

[m-aciek]

There was an unintended bug in my code. I fixed it with 39d01b7.

Python default sorting is ascending and I wanted to get a quotient of sets. The test passed by accident.

1 {'j'}  # I was getting this element
2 {'i', 'j'}

0 set()
1 {'j'}  # after 39d01b7 (fix) I'm getting this (same element, but the logic is fixed)