fun circleInside(
    x1: Double, y1: Double, r1: Double,
    x2: Double, y2: Double, r2: Double
): Boolean {
    val d = trackLength(x1, y1, x2, y2)
    return d <= r2 - r1
}

fun trackLength(x1: Double, y1: Double, x2: Double, y2: Double): Double =
    sqrt(sqr(x1 - x2) + sqr(y1 - y2))