code: public boolean hasUserGrant(Grant grant,String username){
          if (grant == null) throw new IllegalArgumentException("grant cannot be null");
          if (username==null) throw new IllegalArgumentException("username cannot be null");
          Set<String> users = userGrants.get(grant);
          return users!=null &&users.contains(username);
      }
docs: Checks if the user with {@code username} has the specified {@code grant}
  @param grant a {@link com.baasbox.android.Grant}
  @param username a username
  @return