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