A node v is in D-SEP(x,y,G) iff v is not equal to x and there is a collider path between x and v in G such that every vertex on this path is an ancestor of x or y in G.
See p.136 of Sprirtes et al (2000) or Definition 4.1 of Maathuis and Colombo (2015).
dreach(x, y, amat, verbose = FALSE)y must be different from x).M.H. Maathuis and D. Colombo (2015). A generalized back-door criterion. Annals of Statistics 43 1060-1088.
backdoor uses this function;
pag2magAM.