v is in Possible-D-SEP(x,G) iff there is a path p between x and v in G
  such that for every subpath  of p, t is a collider on this
  subpath or  is a triangle in G.
See Spirtes et al (2000) or Definition 3.3 of Colombo et al (2012).
qreach(x, amat, verbose = FALSE)amat[a,b] = 2 and amat[b,a] = 3
    implies a -> b)D. Colombo, M.H. Maathuis, M. Kalisch, T.S. Richardson (2012). Learning high-dimensional directed acyclic graphs with latent and selection variables. Annals of Statistics 40, 294--321.
fci and pdsep which both use
  this function.