Return instance created from merge-origin tc element.

def from_merge_origin(cls, tc):
        """Return instance created from merge-origin tc element."""
        other_tc = tc.tbl.tc(
            tc.row_idx + tc.rowSpan - 1,  # ---other_row_idx
            tc.col_idx + tc.gridSpan - 1  # ---other_col_idx
        )
        return cls(tc, other_tc)