# File ms2body.rb, line 110
  def ms2pc(dt)
    if @nsteps == 0
      @prev_acc = acc
      rk2(dt)
      @old_acc = acc
    else
      old_pos = pos
      jdt = @old_acc - @prev_acc
      @pos += vel*dt + @old_acc*0.5*dt*dt
      new_acc = acc
      jdt = new_acc - @old_acc
      @pos = old_pos + vel*dt + @old_acc*0.5*dt*dt
      @vel += @old_acc*dt + jdt*0.5*dt
      @prev_acc = @old_acc
      @old_acc = new_acc
    end
  end