anchor.diff:          do k=1,ndof
             ik=i0+k
             jk=j0+k
  +          !$omp atomic
             AB(ik)=AB(ik)+A(ij)*B(jk)
           enddo
         enddo
  
positive.code:     allocate(Cmat%item(nnz))
      allocate(Cmat%A(ndof2 * nnz))
      Cmat%A(:) = 0.0d0
      do i = 1, nr
        icnt = 0
        l0 = Cmat%index(i-1)
  
negative.code:       !write(0,*) 'l0,icnt,index(i)',Cmat%index(i-1),icnt,Cmat%index(i)
        if (l0+icnt /= Cmat%index(i)) stop 'ERROR: multiply_mat_mat: unknown error'
      enddo
      !$omp end do
      !$omp end parallel
      call sort_and_uniq_rows(Cmat)
    end subroutine multiply_mat_mat
  
  