docs: def clear(self):
          """
          Calls `_clear` abstract method which must be implemented by descendants.
  
          :raises: GPflowError exception when parent of the node is built.
          """
          parent = self.parent
          if parent is not self and parent.is_built_coherence(self.graph) is Build.YES:
              raise GPflowError('Clear method cannot be started. Upper nodes are built.')
          self._clear()