code: protected boolean shouldEnable( int[] selected ) {
          return selected != null && selected.length >= 1 && (requiredCount == -1 || requiredCount == selected.length);
      }
docs: Determine if the guarded object should be enabled based on the contents
  of the current selection model value.
  
  @param selected The array of selected rows
  @return boolean true if the guarded object should be enabled