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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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()