code: def merge_boundary(graph, src, dst):
      """Call back called before merging 2 nodes.
  
      In this case we don't need to do any computation here.
      """
      pass
  
imports: from skimage import data, segmentation, filters, color
  from skimage import graph
  from matplotlib import pyplot
label_desc: Use image node_id 2 for calling a global function with example usage: merge_boundary(graph, src, dst) without return types