Investigate using constant nodes as drivers

[jcrozum]

I am not sure whether or not the control code properly handles the possibility of overriding constant nodes (see #74). An example that should clarify this is:

A, B
B, A
C, A | D
D, C | E
E, false

The question is: when driving toward A=B=E=0, C=D=1 can E=1 be used to enter the C=D=1 trap space or not? It should be a valid intervention, but I suspect the code will not find it. This needs more testing.