code: public void addNegative(Group group, AccessLevel accessLevel) {
          getAccessList().add(new Access(group, accessLevel, true));
      }
docs: Add the given group to the access list with it's appropriate {@link AccessLevel}.
  Negate the rule.
  <pre>
  ~group = rw
  </pre>
  This means that everybody who is NOT member of the group will
  get the permission (rw).
  @param group The group which will be added to the access list.
  @param accessLevel The accessLevel which will be given to the group.