def _get_layer(self, layer_id):
if layer_id not in self._layer_id_to_layer:<EOL><INDENT>self._svg.setdefault("<STR_LIT:g>", [])<EOL>layer = {<EOL>"<STR_LIT:g>": [],<EOL>"<STR_LIT>": layer_id,<EOL>"<STR_LIT>": layer_id,<EOL>"<STR_LIT>": "<STR_LIT>",<EOL>"<STR_LIT>": "<STR_LIT>"<EOL>}<EOL>self._layer_id_to_layer[layer_id] = layer<EOL>self._svg["<STR_LIT:g>"].append(layer)<EOL><DEDENT>return self._layer_id_to_layer[layer_id]<EOL>
:return: the layer with the :paramref:`layer_id`. If the layer
  does not exist, it is created.
:param str layer_id: the id of the layer