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let draw (dcfg : 'a SimData) cfg signals = 
   ...

  let strFmt = new StringFormat(Enum.combine [StringFormatFlags.NoWrap]) in
  let draw_string (graphics : Graphics) text brush (rect : RectangleF) = 
    graphics.DrawString(text, cfg.font, brush, rect, strFmt) in
  
  let draw_transition
    (graphics : Graphics) (pen : Pen) brush 
    (x_off, y_off) (x_size, y_size) y_margin
    run prev cur nBits = 
    let line (x0 : int) y0 x1 y1 = graphics.DrawLine(pen, x0+x_off, y0+y_off, x1+x_off, y1+y_off) in
    let down  () = line 0 y_margin          0      (y_size-y_margin)    in
    let up    () = line 0 (y_size-y_margin) 0      y_margin             in
    let top   () = line 0 y_margin          x_size y_margin             in
    let bottom() = line 0 (y_size-y_margin) x_size (y_size-y_margin)    in
    let text  r  = 
      let pnt = new PointF(float32_of_int (x_off + cfg.fontMargin), float32_of_int (y_off+y_margin+cfg.fontMargin)) in
      let rect = new SizeF(float32_of_int ((run * cfg.xSize)-(2*cfg.fontMargin)), float32_of_int (y_size-(2*(cfg.yMargin+cfg.fontMargin)))) in
      let text = string_of_int (dcfg.to_int cur) in
      let rendered_width = graphics.MeasureString(text, cfg.font) in
      let text = if rendered_width.Width > rect.Width then "." else text in
      draw_string graphics text brush (new RectangleF(pnt, rect))
    in
    if nBits = 1 then
      match dcfg.to_int prev, dcfg.to_int cur with
      | 0, 0 -> bottom()
      | 1, 0 -> down(); bottom()
      | 0, 1 -> up(); top()
      | 1, 1 -> top()
      | _ -> failwith "not implemented"
    else
      if run = 0
      then (top(); bottom())
      else (up(); top(); bottom(); text run)
  in
  ...

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System.InvalidCastException: Unable to cast object of type 'draw_transition@137[Microsoft.FSharp.Ref[System.Int32]]' to type 'draw_transition@137[System.String]'.
   at DigitalLogic.Waveform.draw_signal@169_3.Invoke(Int32 y_margin, Tuple`4 signal)
   at Microsoft.FSharp.MLLib.List.iteri_aux@47[A](Int32 n, FastFunc`2 f, List x)
   at DigitalLogic.Waveform.draw_signals@183.Invoke(SolidBrush textBrush)
   at Microsoft.FSharp.Idioms.using[W35,B](W35 ie, FastFunc`2 f)
   at Microsoft.FSharp.Idioms.using[W35,B](W35 ie, FastFunc`2 f)