<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def AddNewSpecification(self, identifier):
 """Adds a new format specification. Args: identifier (str):
 format identifier, which should be unique for the store. Returns: FormatSpecification: format specification. Raises: KeyError: if the store already contains a specification with the same identifier. """

    if identifier in self._format_specifications:
      raise KeyError(
          'Format specification {0:s} is already defined in store.'.format(
              identifier))

    self._format_specifications[identifier] = FormatSpecification(identifier)

    return self._format_specifications[identifier]