code: def compute_diff(values1, values2):
      assert len(values1) == len(values2)
      ret = []
      for i in range(len(values1)):
          v1 = values1[i]
          v2 = values2[i]
          if v1 is not None and v2 is not None:
              diff = v1 - v2
          else:
              diff = None
          ret.append(diff)
      return ret
  
label_desc: Use image node_id 1 for calling a global function with example usage: compute_diff(values1, values2) and returns: ret