code: @Override
      public double getValueAt(final int at) {
          if (userPrecAtCutoff.containsKey(at)) {
              int n = 0;
              double prec = 0.0;
              for (U u : userPrecAtCutoff.get(at).keySet()) {
                  double uprec = getValueAt(u, at);
                  if (!Double.isNaN(uprec)) {
                      prec += uprec;
                      n++;
                  }
              }
              if (n == 0) {
                  prec = 0.0;
              } else {
                  prec = prec / n;
              }
              return prec;
          }
          return Double.NaN;
      }
docs: Method to return the precision value at a particular cutoff level.
  
  @param at cutoff level
  @return the precision corresponding to the requested cutoff level