Builds one row per instrument out of every source that knows about it, later sources winning
field by field. A row is a token the picker can offer, so the union is the catalogue and a
holdings source only annotates it: a token nobody holds is still a row, and one nothing named is
a row under its own id.
Builds one row per instrument out of every source that knows about it, later sources winning field by field. A row is a token the picker can offer, so the union is the catalogue and a holdings source only annotates it: a token nobody holds is still a row, and one nothing named is a row under its own id.