code: private void verifyMutuallyExclusive(Collection<Host> A, Collection<Host> B) {
  		
  		Set<Host> left = new HashSet<Host>(A);
  		Set<Host> right = new HashSet<Host>(B);
  		
  		boolean modified = left.removeAll(right);
  		if (modified) {
  			throw new RuntimeException("Host up and down sets are not mutually exclusive!");
  		}
  	}
docs: Helper method to check that there is no overlap b/w hosts up and down.
  @param A
  @param B