code: @Override
      public double getValueAt(final U user, final int at) {
          if (userRecallAtCutoff.containsKey(at) && userRecallAtCutoff.get(at).containsKey(user)) {
              return userRecallAtCutoff.get(at).get(user) / userTotalRecall.get(user);
          }
          return Double.NaN;
      }
docs: Method to return the recall value at a particular cutoff level for a
  given user.
  
  @param user the user
  @param at cutoff level
  @return the recall corresponding to the requested user at the cutoff
  level