pub fn union_keys(a: &[(u32, u32)], b: &[(u32, u32)]) -> Vec<(u32, u32)>Expand description
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.