Counting anything from a fixed set of possibilities, e.g. the number of each type of error in a log file or the number of uses of each word in a lexicon in a body of documents.  It's a special case of other data structures, e.g. hash tables mapping items to integers, but it's good to have because it clearly expresses the programmer's intent and can be optimized.  Sets, after all, are just maps from items to arbitrary values, but they're still useful to have.

 

Aha, it's because you've been using complex data structures!  This one is super simple, but it does capture common patterns.

You're right.  There should also be a bag-unique-elements function, which returns the set of unique elements present in the particular bag.

John, have you already done one of your comprehensive surveys of existing implementations?  I'd love to compare the proposal to what's already available.