(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)))