code: def tree(self, level=0, prefix=''):
          
          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
docs: A string representation of the tree rooted in this node, useful for
          debugging purposes.