docstr: Returns the difference between the current homology group of the
  cell and its parent group
docstr_tok: ["Returns","the","difference","between","the","current","homology","group","of","the","cell","and","its","parent","group"]
code: def homology_group_differential(self):
      """
      Returns the difference between the current homology group of the
      cell and its parent group
      """
      if self.hg_d is None:
          self.hgd = self.hg_n - self.p_hgr
  
      return self.hgd
  
imports: import numpy
  import copy
label_desc: Use image node_id 5 for calling the VertexGroup obj's underlying member method code with example usage: obj.homology_group_differential() and returns: self