Status: IN
Counter provides a complete algebraic toolkit for multiset problems: construction (from iterables), measurement (frequency queries with zero-default), comparison (subtraction as containment test), and combination (intersection as element-wise min), making it the single abstraction sufficient to cover the entire multiset problem class without supplementary data structures.
frequency measurement + subtraction containment + intersection min = algebraically complete for multisets
Depends on (SL): counter-universal-frequency-primitive, counter-subtraction-is-multiset-containment-test