docs: @Nullable
      LabelFragmentSet tryExpandSubs(Variable typeVar, TransactionOLTP tx) {
          if (labels().size() != 1) return null;
  
          Label oldLabel = Iterables.getOnlyElement(labels());
  
          SchemaConcept concept = tx.getSchemaConcept(oldLabel);
          if (concept == null) return null;
  
          Set<Label> newLabels = concept.subs().map(SchemaConcept::label).collect(toSet());
  
          return new AutoValue_LabelFragmentSet(varProperty(), typeVar, ImmutableSet.copyOf(newLabels));
      }