code: def _get_xy_tree(xy, degree):
      
      x, y = xy
      tree = [numpy.array([numpy.ones(x.shape, dtype=int)])]
      for d in range(degree):
          tree.append(numpy.concatenate([tree[-1] * x, [tree[-1][-1] * y]]))
      return tree
docs: Evaluates the entire tree of 2d mononomials.
  
      The return value is a list of arrays, where `out[k]` hosts the `2*k+1`
      values of the `k`th level of the tree
  
          (0, 0)
          (1, 0)   (0, 1)
          (2, 0)   (1, 1)   (0, 2)
            ...      ...      ...