code: public List<String> getDomainObjectTypeNames() {
  		List<DomainObjectType> types = getDomainObjectTypes();
  		List<String> typeNames = new ArrayList<String>(types.size());
  		for (DomainObjectType typ : types) {
  			typeNames.add(typ.getTypeName());
  		}
  		return typeNames;
  	}
docs: answer a list of names of DomainObjectTypes stored in the domain graph
  @return a list of DomainObjectTypes