code: void countNonZeroInR( int[] parent ) {
          TriangularSolver_DSCC.postorder(parent,n,post,gwork);
          columnCounts.process(A,parent,post,countsR);
          nz_in_R = 0;
          for (int k = 0; k < n; k++) {
              nz_in_R += countsR[k];
          }
          if( nz_in_R < 0)
              throw new RuntimeException("Too many elements. Numerical overflow in R counts");
      }
docs: Count the number of non-zero elements in R