ODE in F#

module ODE let isBetween (a1, a2) x =     if a1 < a2 then a1 <= x && x<=a2     else            a2 <= x && x<=a1 let SolveEuler (f:float*float->float) (h:float) (init:float*float) (T:float) =     let t0, _ = init     Seq.unfold (fun ty ->                     let t_, y_ = fst ty, snd ty                     if not (isBetween(t0, T) t_) then None                     else                         Some (ty, (t_ + h, y_ + h*(f ty)))                 ) init let SolveRungeKutta (f:float*float->float) (h:float) (init:float*float) (T:float) =     let t0, _ = init     Seq.unfold (fun ty ->                     let t_, y_ = fst ty, snd ty                                        if not (isBetween(t0, T) t_) then None                     else                         let k1 = f ty                         let k2 = f (0.5*h+t_, y_+0.5*h*k1)                         let k3 = f (0.5*h+t_, y_+0.5*h*k2)                         let k4 = f (    h+t_, y_+h*k3)                         let t,y = t_+h, y_ + h*(k1+2.0*k2+2.0*k3+k4)/6.0                         Some (ty, (t, y))                 ) init