code: private static Collection<Writer> getAllWriters(final Collection<Writer>[][] matrix) {
  		Collection<Writer> writers = Collections.newSetFromMap(new IdentityHashMap<Writer, Boolean>());
  		for (int i = 0; i < matrix.length; ++i) {
  			for (int j = 0; j < matrix[i].length; ++j) {
  				writers.addAll(matrix[i][j]);
  			}
  		}
  		return writers;
  	}
docs: Collects all writer instances from a matrix of writers.
  
  @param matrix
  All writers
  @return Collection that contains each writer only once