# File ms4body_old.rb, line 97
  def ms2(dt)
    if @nsteps == 0
      @prev_acc = acc
      rk2(dt)
    else
      old_acc = acc
      jdt = old_acc - @prev_acc
      @pos += vel*dt + old_acc*0.5*dt*dt
      @vel += old_acc*dt + jdt*0.5*dt
      @prev_acc = old_acc
    end
  end