docs: def getValidItem(self, specfile, identifier):
          """Returns a ``Sii`` instance from ``self.container`` if it is valid,
          if all elements of ``self.container[specfile][identifier] are
          ``Sii.isValid == False`` then ``None`` is returned.
  
          :param specfile: a ms-run file name
          :param identifier: item identifier ``Sii.id``
  
          :returns: ``Sii`` or ``None``
          """
          for item in self.container[specfile][identifier]:
              if item.isValid:
                  return item
          else:
              return None