step :: Probe -> Probe
step probe = probe & pos .~ (probe ^. pos ^+^ probe ^. vel) & vel .~ vel'
where vel' = V2 (max 0 (vx - 1)) (vy - 1)
- V2 vx vy = _vel
+ V2 vx vy = probe ^. vel
-- v = probe ^. vel
-- vel' = v & _x .~ (max 0 ((v ^. _x) - 1)) & _y .~ ((v ^. _y) - 1)