node(a; b; c; d).
depends(b,a).
depends(c,a).
depends(d, a).
depends(d, c). 

level(X, 0) :- not depends(X, _); node(X).
level(X, LEVEL):- LEVEL = #max{L+1, level(Y, L): level(Y, L), depends(X, Y)}; node(X); LEVEL > 0.