private static boolean isChild(Lop a,Lop b,HashMap<Long,Integer> IDMap){
  int bID=IDMap.get(b.getID());
  return a.get_reachable()[bID];
}

private static boolean isChild ( Lop a , Lop b , HashMap < Long , Integer > IDMap ) { int bID = IDMap . get ( b . getID ( ) ) ; return a . get_reachable ( ) [ bID ] ; }
check to see if a is the child of b ( i . e . , there is a directed path from a to b )
