Status: IN
Counter subtraction serves as a multiset containment test across the repo: the drop-nonpositive semantics of `Counter.__sub__` mean that an empty result after `A - B` is equivalent to B containing at least as many of every element as A, and `not (A - B)` is the idiomatic one-expression sub-multiset check — replacing explicit key-by-key iteration with a single algebraic operation.
Depth-1 unification: three base beliefs describe mechanism (drops nonpositive), consequence (empty = containment), and application (subset check) of the same Counter algebraic property
Depends on (SL): counter-subtraction-as-subset-check, counter-sub-empty-means-containment, counter-subtraction-drops-nonpositive