wallType (Qplus, Q)
every wall can be represented uniquely by a partition of the vertices Q0 of Q into two sets Qplus and Qminus. We denote the wall W by the subset of vertices Qplus used for defining it.
The type of the wall is defined as (t+,t) where t^+ is the number of arrows starting Qplus and ending in Qminus, and t is the number of arrows starting Qminus and ending in Qplus.

The object wallType is a method function.