code: def _merge_meta_data(cls, first: "HistogramBase", second: "HistogramBase") -> dict:
          
          keys = set(first._meta_data.keys())
          keys = keys.union(set(second._meta_data.keys()))
          return {key:
                  (first._meta_data.get(key, None) if first._meta_data.get(key, None) == second._meta_data.get(key, None) else None)
                  for key in keys}
docs: Merge meta data of two histograms leaving only the equal values.
  
          (Used in addition and subtraction)