balance is the only category_type whose subtype column is not orthogonal to
category. Measured against the crosswalk: 5 of 6 expenditure subtypes and 1 of
7 revenue subtypes span more than one category, but 0 of 5 balance subtypes do.
Balance is a strict tree -- Fund Balances = {general}, Retirement System
Holdings = {employee_retirement}, Insurance Trust Balances = the three trust
subtypes.
Exposing both arguments would admit no useful combination: of the 15 pairs, 3
are redundant and 12 are guaranteed empty for every government in every year,
failing as an empty tibble that reads as "holds none" rather than as a
contradiction.
Dropping it also keeps the verb aligned -- no uscogdata verb exposes a subtype
argument; the API layers its own subtype row filter on top, which cog-api#26
can do for /balances. #25's one-filter requirement is still met, since
category = "Fund Balances" is exactly W01/W31/W61.
Adds two tests: that one-filter equivalence, and an assertion that the
subtype -> category tree holds, so an upstream change making category lossy
fails here rather than in a user's analysis.
jared
9f9d40e1c3