code: public OSchemaHelper oIndex(String name, INDEX_TYPE type, String... fields)
  	{
  		checkOClass();
  		lastIndex = lastClass.getClassIndex(name);
  		if(lastIndex==null)
  		{
  			lastIndex = lastClass.createIndex(name, type, fields);
  		}
  		else
  		{
  			//We can't do something to change type and fields if required
  		}
  		return this;
  	}
docs: Create {@link OIndex} on a set of fields if required
  @param name name of an index
  @param type type of an index
  @param fields fields to create index on
  @return this helper