For each SLT, there must exist at least one P_SLP such that ran
P_SLP=SLT.
For each SLET, there must exist at least one P_SL such that ran
P_SLP=SLET.
For each SLT, there must exist at least one SLET.
For each VT, there must exist at least one P_SLE such that ran P_SLP=VT.
For each SLET, there must exist at least one VT.
No two different VTs must be connected to the same VC.