openingsInDocument function
host's openings in doc: the live openings (the engine's object
rule, asked through live_objects.dart) whose host is host, ascending
by handle, with their parameters, as openingsInView reads them in a
regeneration.
Implementation
List<(Handle, OpeningParams)> openingsInDocument(
DraftDocument doc, Handle host) =>
[
for (final h in liveObjectsOf<OpeningParams>(doc))
if (doc.components.get<OpeningParams>(h)! case final o
when o.host == host)
(h, o),
];