 BTKCROPACQUISITION(H, STARTAT) keeps all the frames from the frame STARTAT.
  The function shifts also the events' frame/time by adding the difference 
  between the new first frame and the old one. For this adaptation, the sample
  frequency must be non-null.
  The acquisition is represented by the handle H, obtained by the use of a 
  btk* function.

  BTKCROPACQUISITION(H, STARTAT, NUMFRAMES) keeps NUMFRAMES frames starting
  from the frame STARTAT.</pre></div>

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