docs: public static DMatrixRBlock identity(int numRows, int numCols, int blockLength ) {
          DMatrixRBlock A = new DMatrixRBlock(numRows,numCols,blockLength);
  
          int minLength = Math.min(numRows,numCols);
  
          for( int i = 0; i < minLength; i += blockLength ) {
              int h = Math.min(blockLength,A.numRows-i);
              int w = Math.min(blockLength,A.numCols-i);
  
              int index = i*A.numCols + h*i;
  
              int m = Math.min(h,w);
              for( int k = 0; k < m; k++ ) {
                  A.data[index + k*w + k ] = 1;
              }
          }
  
          return A;
      }