PC algorithm graph matrix syntax

[Kenneth Lee]

Hi,

I am quite new to this repo. I have a question regarding the documentation about the following syntax for the output in pc.py. Why would there be a need to distinguish i---j and i<->j in the output of CPDAG adjacency matrix? Would someone mind to explain?

                    cg.G.graph[i,j] = cg.G.graph[j,i] = -1 indicates i --- j,
                    cg.G.graph[i,j] = cg.G.graph[j,i] = 1 indicates i <-> j.