CHEMIN_AB = CHEMIN_AB[0][0],
CHEMIN_AB[na:T][nb:T]= (
	when (nb ==0) trainA[i:ID].entreeAB -> CHEMIN_AB[na+1][nb]|
	trainA[j:ID].sortieAB -> CHEMIN_AB[na-1][nb]|
	when (na ==0) trainB[i:ID].entreeAB -> CHEMIN_AB[na][nb+1]|
	trainB[j:ID].sortieAB -> CHEMIN_AB[na][nb-1]).