code: String[] concatenateExtends( String[] selectors, boolean isReference ) {
          selectorList.clear();
          for( String selector : selectors ) {
              concatenateExtendsRecursive( selector, isReference, selector );
          }
  
          if( isReference ) {
              return selectorList.toArray( new String[selectorList.size()] );
          }
  
          if( selectorList.size() > 0 ) {
              for( String selector : selectors ) {
                  selectorList.remove( selector ); // remove duplicates
              }
              if( selectorList.size() > 0 ) {
                  int off = selectors.length;
                  selectors = Arrays.copyOf( selectors, off + selectorList.size() );
                  for( String str : selectorList ) {
                      selectors[off++] = str;
                  }
              }
          }
          return selectors;
      }
docs: Add to the given selectors all possible extends and return the resulting selectors.
  
  @param selectors
  current selectors
  @param isReference
  if the current rule is in a less file which was import with "reference" keyword
  @return the selectors concatenate with extends or the original if there are no etends.