predicates
nondeterm reys(symbol, symbol)
nondeterm avia2(symbol, symbol)

clauses
reys(moscow, saint_pitersburg).
reys(moscow, omsk).
reys(omsk, saint_pitersburg).
reys(moscow, perm).
reys(perm, omsk).
reys(perm, kipr).
reys(kipr, vena).
reys(moscow, kipr).

avia2(X,Z):-not(reys(X,Z)),reys(X,Y), reys(Y,Z).
goal
avia2(moscow, kipr).