(bind-func fluid-cube-add-velocity
(lambda (cube:fluidcube* x y amount-x:double amount-y:double)
(let ((N (tref cube 0))
(idx (+ x (* y N)))
(_Vx (tref cube 6))
(_Vy (tref cube 7)))
(pset! _Vx idx (+ amount-x (pref _Vx idx)))
(pset! _Vy idx (+ amount-y (pref _Vy idx)))
cube)))