Status: IN
Counter's algebraic completeness — encompassing construction from iterables, frequency measurement with zero-default, comparison via subtraction, and containment via drop-nonpositive semantics — is the specific mechanism that makes hash-based preprocessing universal: every hash-then-stream solution's preprocessing phase reduces to a composition of Counter's algebraic operations, and Counter's closure under these operations guarantees the preprocessing output is always a valid input to the streaming phase.
Counter's closed algebra is why hash preprocessing covers all frequency and multiset problems
Depends on (SL): counter-is-complete-multiset-algebra, hash-preprocessing-universal-first-step