permute_umat_inds Function

    function permute_umat_inds(a, b, c, d) result(inds)
        integer, intent(in) :: a, b, c, d
        integer(int64) :: inds(num_ex)

        integer :: ct

        inds = 0
        ! These are adjoint to each other, they are the same in LMat, but not UMat
        inds(1) = UMatInd(a, b, c, d)
        inds(2) = UMatInd(c, b, a, d)
        inds(3) = UMatInd(a, d, c, b)
        inds(4) = UMatInd(c, d, a, b)

        ! All terms are only counted if they are different from a previously occuring index
        ! This approach might be less performant than competing ways of doing this check,
        ! but it is very clear and less prone to bugs. If this proves to be a performance
        ! bottleneck, it can be changed to a faster implementation later on
        do ct = 2, num_ex
            ! If the index already appeared, delete it
            if(any(inds(ct) == inds(1:ct - 1))) inds(ct) = 0
        end do
    end function permute_umat_inds