You have N groups of something (persons, sales units, cars, tools, whatever). Find the values that are present in all of them. This problem comes up, all the time, in some form.
With sets it's trivial. Take the first set, and calculate a cascading intersection with every other set. At the end you're left with only the values that appeared in every one. In very clear python the code might look like this:
common = ALL_SETS
for _set in ALL_SETS[1:]:
common = common.intersection(_set)
Btw, in the above code example I have deliberately omitted a trivial optimisation. Let's leave that as an exercise for the reader. :)
return functools.reduce(lambda x,y: x&y, ALL_SETS)