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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _checkpoint(self, stage):
 """ Decide whether to stop processing of a pipeline. This is the hook A pipeline can report various "checkpoints" as sort of status markers that designate the logical processing phase that's just been completed. The initiation of a pipeline can preordain one of those as a "stopping point" that when reached, should stop the pipeline's execution. :param pypiper.Stage | str stage: Pipeline processing stage/phase just completed. :return bool: Whether a checkpoint was created (i.e., whether it didn't already exist) :raise ValueError: If the stage is specified as an absolute filepath, and that path indicates a location that's not immediately within the main output folder, raise a ValueError. """


        # For null stage, short-circuit and indicate no file write.
        # This handles case in which we're timestamping prospectively and
        # previously weren't in a stage.
        if stage is None:
            return False

        try:
            is_checkpoint = stage.checkpoint
        except AttributeError:
            # Maybe we have a raw function, not a stage.
            if hasattr(stage, "__call__"):
                stage = stage.__name__
            else:
                # Maybe we have a stage name not a Stage.
                # In that case, we can proceed as-is, with downstream
                # processing handling Stage vs. stage name disambiguation.
                # Here, though, warn about inputs that appear filename/path-like.
                # We can't rely on raw text being a filepath or filename,
                # because that would ruin the ability to pass stage name rather
                # than actual stage. We can issue a warning message based on the
                # improbability of a stage name containing the '.' that would
                # be expected to characterize the extension of a file name/path.
                base, ext = os.path.splitext(stage)
                if ext and "." not in base:
                    print("WARNING: '{}' looks like it may be the name or path of "
                          "a file; for such a checkpoint, use touch_checkpoint.".
                          format(stage))
        else:
            if not is_checkpoint:
                print("Not a checkpoint: {}".format(stage))
                return False
            stage = stage.name

        print("Checkpointing: '{}'".format(stage))
        if os.path.isabs(stage):
            check_fpath = stage
        else:
            check_fpath = checkpoint_filepath(stage, pm=self)
        return self._touch_checkpoint(check_fpath)