Make sure collisions preserve roll direction.

sdr> /program=Program.PLUS; setFormation(Formation.SQUARED_SET)
|      3Gv  3Bv
| 
| 4B>            2G<
| 
| 4G>            2B<
| 
|      1B^  1G^
sdr> heads lead left
| 4B>  1G<  3B>  2G<
| 
| 4G>  1B<  3G>  2B<
sdr> trade and roll
| 4Gv  1Bv  3Gv  2Bv
| 
| 4B^  1G^  3B^  2G^
sdr> do half of a trade and roll
| 4G^  3G^
| 
| 1B^  2B^
| 
| 4Bv  3Bv
| 
| 1Gv  2Gv
sdr> /exit