code: def _merge_meta(left, right, result, clean=True):
          
          # Copies are returned because they need some clean-up below.
          left = BaseSpectrum._get_meta(left)
          right = BaseSpectrum._get_meta(right)
  
          # Remove these from going into result to avoid mess.
          #   header = FITS header metadata
          #   expr = ASTROLIB PYSYNPHOT expression
          if clean:
              for key in ('header', 'expr'):
                  for d in (left, right):
                      if key in d:
                          del d[key]
  
          mid = metadata.merge(left, right, metadata_conflicts='silent')
          result.meta = metadata.merge(result.meta, mid,
                                       metadata_conflicts='silent')
docs: Merge metadata from left and right onto results.
  
          This is used during class initialization.
          This should also be used by operators to merge metadata after
          creating a new instance but before returning it.
          Result's metadata is modified in-place.
  
          Parameters
          ----------
          left, right : number, `BaseSpectrum`, or `~astropy.modeling.models`
              Inputs of an operation.
  
          result : `BaseSpectrum`
              Output spectrum object.
  
          clean : bool
              Remove ``'header'`` and ``'expr'`` entries from inputs.