static L<RuleEngine2.Rule> gazelle_possibleNextRulesForPreContext(LS preContext, O... _) {
  GazelleEvalContext ctx = gazelle_newContextFromParams(_);
  gazelle_dropRulesNotOnInput(ctx);
  ctx.engine.dropRulesWhere(r -> !empty(r.purpose));
  ctx.engine.dropRulesWhere(r -> gazelle_historyLevelRequired(r) > l(preContext)+1);
  // TODO: filter more
  ret ctx.engine.rules;
}