code: public String createContainingValue(String filename, String key) {
          Object data = value(key);
          if (data == null) {
              throw new SlimFixtureException(false, "No value for key: " + key);
          }
          return createContaining(filename, data);
      }
docs: Creates new file, containing value 'key'.
  @param filename name of file to create.
  @param key key whose value should be used to generate the file.
  @return file created.