docs: public Collection<JcrNodeDefinition> getMandatoryChildNodeDefinitions( Name primaryType,
                                                                             Set<Name> mixinTypes ) {
          if (mixinTypes.isEmpty()) {
              return mandatoryChildrenNodeTypes.get(primaryType);
          }
          Set<JcrNodeDefinition> defn = new HashSet<JcrNodeDefinition>();
          defn.addAll(mandatoryChildrenNodeTypes.get(primaryType));
          for (Name mixinType : mixinTypes) {
              defn.addAll(mandatoryChildrenNodeTypes.get(mixinType));
          }
          return defn;
      }