static ItIt<GazelleRevisitedLine> gazelle_revisitAllLinesInChannel(long channelID, F0<GazelleEvalContext> contextMaker, O... _) {
  L<GazelleLine> lines = dm_discord_linesInChannel(channelID);
  new LS preContext;
  ItIt<GazelleRevisitedLine> it = mapI(lines, line ->
    nu GazelleRevisitedLine(+line,
      tree := dm_gazelle_reasonAboutChatInput_v2(
        line.user, line.text, paramsPlus(_, ctx := contextMaker!, +preContext)));
  if (boolPar nemptyOnly(_))
    it = filterIterator(it, func(GazelleRevisitedLine l) -> bool { nempty(l.tree) });
  ret it;
}

Began life as a copy of #1022083