code: def split_by(self, layer, sep=' '):
          
          if not self.is_tagged(layer):
              self.tag(layer)
          return self.split_given_spans(self.spans(layer), sep=sep)
docs: Split the text into multiple instances defined by elements of given layer.
  
          The spans for layer elements are extracted and feed to :py:meth:`~estnltk.text.Text.split_given_spans`
          method.
  
          Parameters
          ----------
          layer: str
              String determining the layer that is used to define the start and end positions of resulting splits.
          sep: str (default: ' ')
              The separator to use to join texts of multilayer elements.
  
          Returns
          -------
          list of Text