Merge ``adjustments`` with existing adjustments, handling index
        collisions according to ``method``.

        Parameters
        ----------
        adjustments : dict[int -> list[Adjustment]]
            The mapping of row indices to lists of adjustments that should be
            appended to existing adjustments.
        method : {'append', 'prepend'}
            How to handle index collisions. If 'append', new adjustments will
            be applied after previously-existing adjustments. If 'prepend', new
            adjustments will be applied before previously-existing adjustments.

def update_adjustments(self, adjustments, method):
        """
        Merge ``adjustments`` with existing adjustments, handling index
        collisions according to ``method``.

        Parameters
        ----------
        adjustments : dict[int -> list[Adjustment]]
            The mapping of row indices to lists of adjustments that should be
            appended to existing adjustments.
        method : {'append', 'prepend'}
            How to handle index collisions. If 'append', new adjustments will
            be applied after previously-existing adjustments. If 'prepend', new
            adjustments will be applied before previously-existing adjustments.
        """
        try:
            merge_func = _merge_methods[method]
        except KeyError:
            raise ValueError(
                "Invalid merge method %s\n"
                "Valid methods are: %s" % (method, ', '.join(_merge_methods))
            )

        self.adjustments = merge_with(
            merge_func,
            self.adjustments,
            adjustments,
        )