Returns absolute reference code for key.

        Parameters
        ----------

        cursor: 3-tuple of Integer
        \tCurrent cursor position
        ref_key: 3-tuple of Integer
        \tAbsolute reference key

def _get_relative_reference(self, cursor, ref_key):
        """Returns absolute reference code for key.

        Parameters
        ----------

        cursor: 3-tuple of Integer
        \tCurrent cursor position
        ref_key: 3-tuple of Integer
        \tAbsolute reference key

        """

        magics = ["X", "Y", "Z"]

        # mapper takes magic, key, ref_key to build string
        def get_rel_key_ele(cursor_ele, ref_key_ele):
            """Returns relative key suffix for given key and reference key"""

            # cursor is current cursor position
            # ref_key is absolute target position

            diff_key_ele = ref_key_ele - cursor_ele

            if diff_key_ele == 0:
                return u""

            elif diff_key_ele < 0:
                return u"-" + str(abs(diff_key_ele))

            elif diff_key_ele > 0:
                return u"+" + str(diff_key_ele)

            else:
                msg = _("{key} seems to be no Integer")
                msg = msg.format(key=diff_key_ele)
                raise ValueError(msg)

        key_strings = []

        for magic, cursor_ele, ref_key_ele in zip(magics, cursor, ref_key):
            key_strings.append(magic +
                               get_rel_key_ele(cursor_ele, ref_key_ele))

        key_string = u", ".join(key_strings)

        return u"S[" + key_string + u"]"