Calls `_clear` abstract method which must be implemented by descendants.

        :raises: GPflowError exception when parent of the node is built.

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()