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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _de_casteljau_one_round(nodes, degree, lambda1, lambda2, lambda3):
 r"""Performs one "round" of the de Casteljau algorithm for surfaces. .. note:: There is also a Fortran implementation of this function, which will be used if it can be built. .. note:: This is a helper function, used by :func:`make_transform` and :func:`_specialize_surface` (and :func:`make_transform` is **only** used by :func:`_specialize_surface`). Converts the ``nodes`` into a basis for a surface one degree smaller by using the barycentric weights: .. math:: q_{i, j, k} = \lambda_1 \cdot p_{i + 1, j, k} + \lambda_2 \cdot p_{i, j + 1, k} + \lambda_2 \cdot p_{i, j, k + 1} .. note: For degree :math:`d`, the number of nodes should be :math:`(d + 1)(d + 2)/2`, but we don't verify this. Args: nodes (numpy.ndarray):
 The nodes to reduce. degree (int):
 The degree of the surface. lambda1 (float):
 Parameter along the reference triangle. lambda2 (float):
 Parameter along the reference triangle. lambda3 (float):
 Parameter along the reference triangle. Returns: numpy.ndarray: The converted nodes. """

    dimension, num_nodes = nodes.shape
    num_new_nodes = num_nodes - degree - 1
    new_nodes = np.empty((dimension, num_new_nodes), order="F")
    index = 0
    # parent_i1 = index + k
    # parent_i2 = index + k + 1
    # parent_i3 = index + degree + 1
    parent_i1 = 0
    parent_i2 = 1
    parent_i3 = degree + 1
    for k in six.moves.xrange(degree):
        for unused_j in six.moves.xrange(degree - k):
            # NOTE: i = (degree - 1) - j - k
            new_nodes[:, index] = (
                lambda1 * nodes[:, parent_i1]
                + lambda2 * nodes[:, parent_i2]
                + lambda3 * nodes[:, parent_i3]
            )
            # Update all the indices.
            parent_i1 += 1
            parent_i2 += 1
            parent_i3 += 1
            index += 1
        # Update the indices that depend on k.
        parent_i1 += 1
        parent_i2 += 1
    return new_nodes