code: public final String getVariableNameFromIndex(int num) {
      int index = getVariableIndex(num);
      if (index >= 0) {
        return names[index];
      } else {
        return null;
      }
    }
docs: Gets the name of the variable whose numerical index is
  {@code index}. Returns {@code null} if no such variable
  exists.
  
  @return