@Adám This is probably obvious to y'all, but thinking about this some more, the ≤ actually directly expresses the "is found in" idea specified in the question, and isn't just some golf trick that conveniently happens to give the right result in all four cases. If you think of booleans as "the number of times", then ≤ is literally "does not happen more times", i.e., "if it happens on the left, it must also happen on the right".
