new_contents: #! /usr/bin/env python
  import numpy as np
  
  from nose.tools import assert_is, assert_is_instance
  from numpy.testing import assert_array_equal
  
  from landlab.grid.diagonals import create_nodes_at_diagonal
  
  
  def test_nodes_at_diagonal():
      """Test tail and head nodes of diagonals."""
      diagonals = create_nodes_at_diagonal((2, 2))
      assert_array_equal(diagonals, [[0, 3], [1, 2]])
  
      diagonals = create_nodes_at_diagonal((4, 3))
      assert_is_instance(diagonals, np.ndarray)
      assert_array_equal(diagonals, [[0,  4], [1, 3], [1,  5], [2,  4],
                                     [3,  7], [4, 6], [4,  8], [5,  7],
                                     [6, 10], [7, 9], [7, 11], [8, 10]])
  
  
  def test_nodes_at_diagonal_1d():
      """Test nodes at diagonals for 1d grid."""
      diagonals = create_nodes_at_diagonal((1, 2))
      assert_array_equal(diagonals, np.array([], dtype=int).reshape((0, 2)))
  
      diagonals = create_nodes_at_diagonal((4, 1))
      assert_array_equal(diagonals, np.array([], dtype=int).reshape((0, 2)))
  
  
  def test_nodes_at_diagonal_out_keyword():
      """Test out keyword for nodes_at_diagonal."""
      buffer = np.empty((4, 2), dtype=int)
      diagonals = create_nodes_at_diagonal((3, 2), out=buffer)
  
      assert_is(buffer, diagonals)
      assert_array_equal(diagonals, [[0, 3], [1, 2], [2, 5], [3, 4]])
  
subject: Add some unit tests for the diagonals.