code: void addPosition( String filename, int line, int column ) {
          LessFilePosition pos = new LessFilePosition( filename, line, column );
          if( !positions.contains( pos ) ) {
              this.positions.add( pos );
          }
      }
docs: Add a position to the less file stacktrace
  @param filename the less file, can be null if a string was parsed
  @param line the line number in the less file
  @param column the column in the less file