def add_cube_verticies(cube_str, x, y, z, point_size):
    """
    Appends the described cube verticies to the given string.

    Parameters
    ----------
    cube_str : str
        the curent string representation of the cube
    x : float
        the x position of the cube
    y : float
        the y position of the cube
    z : float
        the z position of the cube
    point_size : float
        the size of the cube
    """
    vert_permutations = [
                (1,-1,-1),
                (1,-1,1),
                (-1,-1,1),
                (-1,-1,-1),
                (1,1,-1),
                (1,1,1),
                (-1,1,1),
                (-1,1,-1)
            ]
    for perm in vert_permutations:
        cube_str += "v %s %s %s\n" % (
                    x + (point_size * perm[0]),
                    y + (point_size * perm[1]),
                    z + (point_size * perm[2])
                )
    return cube_str