code: def set(self, tag, value):
          
          full_tag = 'Xmp.pyctools.' + tag
          self.data[full_tag] = value
docs: Set a metadata value.
  
          Each metadata value is referenced by a ``tag`` -- a short
          string such as ``'xlen'`` or ``'audit'``. In the sidecar file
          these tag names are prepended with ``'Xmp.pyctools.'``, which
          corresponds to a custom namespace in the XML file.
  
          :param str tag: The tag name.
  
          :param str value: The metadata value.