Module keys 

Coordinate-key projection and union: the key arithmetic of every dimensional join.

A coordinate key is a (category, item) list sorted by category with no category twice (sorted), as every engine CoordKey is.

Functions

project

k restricted to the categories in keep: exactly the pairs whose category is kept, in their order (a filter), so a sorted key projects to a sorted key.

union_keys

The union of two sorted keys, by merge: sorted (so no category twice); every pair of a; every pair of b whose category a lacks (all of b when the two agree on their shared categories, as join partners do); and nothing else. On a shared category a’s item wins.