code: def _mergeCategories(a, b):
      if a == "0":
          return b
      if b == "0":
          return a
      if a == b:
          return a
      return None
  
imports: from fontTools.cffLib import maxStackLimit
label_desc: Use image node_id 11 for calling a global function with example usage: _mergeCategories(a, b) and returns: None, b, a, a