When two synchronised LMs pass through the collider, they either rebound or coalesce, according to their velocity at impact. If the LMs coalesce, then the new LM travels straight down the only AB path, which can be considered to be the carry output. Alternatively if the LMs rebound, as in the SSM, then there are two AB paths. One of these paths is then considered to be the carry output, and the other is discarded (the choice between the two AB paths is arbitrary in this case).