pub fn vel_transform(q: f64, transform_type: i32, x: f64, y: f64, vx: f64, vy: f64) -> (f64, f64) {
let mu: f64 = q/(1.0+q);
let rad: f64 = (x*x + y*y).sqrt();
let vkep: f64 = 1.0/((1.0 + q)*rad).sqrt();
match transform_type {
1 => (vx - y, vy + x - mu),
2 => (-vkep*y/rad, vkep*x/(rad-mu)),
3 => (vx, vy),
_ => panic!("{} is not a valid transform_type.", transform_type),
}
}