def tree(self, level=0, prefix=''):
        """
        A string representation of the tree rooted in this node, useful for
        debugging purposes.
        """
        text = '%s%s\n' % (prefix, self.value.text)
        prefix = '%s└── ' % ('    ' * level)
        if self._lesser:
            text += self._lesser.tree(level+1, prefix)
        if self._greater:
            text += self._greater.tree(level+1, prefix)
        return text