code: def keys(self):
          
          keys = []
          for key, value in self._values.items():
              if isinstance(value, (FlatDict, dict)):
                  nested = [self._delimiter.join([key, k]) for k in value.keys()]
                  keys += nested if nested else [key]
              else:
                  keys.append(key)
          return sorted(keys)
docs: Return a copy of the flat dictionary's list of keys.
          See the note for :meth:`flatdict.FlatDict.items`.
  
          :rtype: list