code: def _pfp__add_child(self, name, child, stream=None):
          
          res = super(Union, self)._pfp__add_child(name, child)
          self._pfp__buff.seek(0, 0)
          child._pfp__build(stream=self._pfp__buff)
          size = len(self._pfp__buff.getvalue())
          self._pfp__buff.seek(0, 0)
  
          if stream is not None:
              curr_pos = stream.tell()
              stream.seek(curr_pos-size, 0)
  
          return res
docs: Add a child to the Union field
  
          :name: The name of the child
          :child: A :class:`.Field` instance
          :returns: The resulting field