code: def stack_boundaries(boundaries):
      
      if len(boundaries['holes']) == 0:
          return boundaries['shell']
      result = np.vstack((boundaries['shell'],
                          np.vstack(boundaries['holes'])))
      return result
docs: Stack the boundaries of a polygon into a single
      (n, 2) list of vertices.
  
      Parameters
      ------------
      boundaries: dict, with keys 'shell', 'holes'
  
      Returns
      ------------
      stacked: (n, 2) float, list of vertices