code: public void eachNonZero(MatrixProcedure procedure) {
          MatrixIterator it = nonZeroIterator();
  
          while (it.hasNext()) {
              double x = it.next();
              int i = it.rowIndex();
              int j = it.columnIndex();
              procedure.apply(i, j, x);
          }
      }
docs: Applies given {@code procedure} to each non-zero element of this matrix.
  
  @param procedure the matrix procedure