docs: def edge_length_check(length, edge):
      """ Raises error if length is not in interval [0, edge.length] """
      try:
          assert 0 <= length <= edge.length
      except AssertionError:
          if length < 0:
              raise TreeError('Negative edge-lengths are disallowed')
          raise TreeError(
              'This edge isn\'t long enough to prune at length {0}\n'
              '(Edge length = {1})'.format(length, edge.length))