def get_layer(self, class_: Type[L], became: bool=True) -> L:
try:<EOL><INDENT>return self._index[class_][<NUM_LIT:0>]<EOL><DEDENT>except KeyError:<EOL><INDENT>if became:<EOL><INDENT>return self._transformed[class_][<NUM_LIT:0>]<EOL><DEDENT>else:<EOL><INDENT>raise<EOL><DEDENT><DEDENT>
Return the first layer of a given class. If that layer is not present,
then raise a KeyError.

:param class_: class of the expected layer
:param became: Allow transformed layers in results