def point_between(point_1, point_2, prop=0.5):
    """Calculates coordiantes of the middle between two points in R^3."""
    x_1, y_1, z_1 = point_1
    x_2, y_2, z_2 = point_2
    return x_1 * (1 - prop) + x_2 * prop, y_1 * (1 - prop) + y_2 * prop, z_1 * (1 - prop) + z_2 * prop