anchor.diff:      ! tau_ij = -2*nu*S_ij
       do m = 1, nq
           do l = 1, nq
  -            tauij(:,:,:,l,m) = -2.0*nu_sgs*tauij(:,:,:,l,m)
  +            tauij(:,:,:,l,m) = 2.0*nu_sgs*0.5*( duidxj(:,:,:,l,m) + duidxj(:,:,:,m,l) )
           end do
       end do
   
  
positive.code:                 if (D_sigma<0.0 .or. isnan(D_sigma)) D_sigma = 0.0
  
                  ! compute nu_sgs = (C*delta)^2 D_sigma
                  nu_sgs(i,j,k) = (C_sigma*delta)**2*D_sigma 
  
              end do
          end do
  
negative.code:     C_sigma = 1.35 ! parameter from Nicoud et al.
  
      tauij = 0.0
      nu_sgs = 0.0
  
      do k = 1, nzp
          do j = 1, nypl
  