anchor.diff:  
       call getri(working, ipiv)
       inv = working
  -  end subroutine invMat
  +  end subroutine invSymMat
   
     subroutine calcVHT(vHT, Bx, By, Bz, vx, vy, vz)
       implicit none
  
positive.code:     logical, intent (in), optional :: isSingularExpected
  
      double precision, dimension (3, 3) :: working
      double precision, dimension (3) :: ipiv
      logical :: isNotComplainAboutSingular
  
      integer :: info
  
negative.code:     end do
      end do
      sumKm_dotVm = sumKm_dotVm / nGrid
      KZero = sumKm / nGrid
      call invSymMat(invKZero, KZero, .true.)
      res = matmul(invKZero, sumKm_dotVm)
  
  