A confusion matrix is four set intersections. With $A$ the predicted positives and $B$ the actual positives:

• true positive: $A \cap B$
• false positive: $A \cap B^c$
• false negative: $A^c \cap B$
• true negative: $A^c \cap B^c$

Return a dict of the four counts under keys "tp", "fp", "fn", "tn".

solve({1,2,3}, {2,3,4}, {1,2,3,4,5})
->  {"tp": 2, "fp": 1, "fn": 1, "tn": 1}