1 2                               1 2             r b b r                           b r r b           r         b                       b         b          r           r                     b           r         r             r                   w             r        b               r                 w               w       b                 b               r                 r       b                 b               b                 b       b                 b               r                 b        r               r                 b               r         b             r                   r             r          b           r                     r           r            r       r                         r       b              r b r                             r r w             Figure A                         Figure B                         r red bead                         b blue bead                         w white bead