code: private static void addWriter(final Writer writer, final Collection<Writer>[][] matrix, final int tagIndex, final Level level) {
  		for (int levelIndex = level.ordinal(); levelIndex < Level.OFF.ordinal(); ++levelIndex) {
  			Collection<Writer> collection = matrix[tagIndex][levelIndex];
  			if (collection == null) {
  				collection = new ArrayList<Writer>();
  				matrix[tagIndex][levelIndex] = collection;
  			}
  			collection.add(writer);
  		}
  	}
docs: Adds a writer to a well-defined matrix. The given writer will be added only at the given tag index for severity
  levels equal or above the given severity level.
  
  @param writer
  Writer to add
  @param matrix
  Well-defined two-dimensional matrix
  @param tagIndex
  Represents the tag (first dimension of matrix)
  @param level
  Represents the severity level (second dimension of matrix)