code: private CssFormatter copy( @Nullable StringBuilder output ) {
          try {
              CssFormatter formatter = (CssFormatter)clone();
              formatter.output = output == null ? state.pool.get() : output;
              formatter.insets = state.pool.get();
              formatter.insets.append( insets );
              return formatter;
          } catch( CloneNotSupportedException ex ) {
              throw new LessException( ex );
          }
      }
docs: Create a new formatter for a single rule with optional output.
  
  @param output
  optional target
  @return a new formatter
  @throws LessException
  if any error occur.