inv_k_vec Function

    function inv_k_vec(this, k) result(k_inv)
        class(lattice) :: this
        integer, intent(in) :: k(3)
        integer :: k_inv(3)
#ifdef DEBUG_
        character(*), parameter :: this_routine = "inv_k_vec"
#endif

        ASSERT(allocated(this%sym_to_k))
        ASSERT(allocated(this%inv_table))
        ASSERT(allocated(this%k_to_sym))

        k_inv = this%sym_to_k(this%inv_table(this%k_to_sym(k(1), k(2), k(3))), :)

    end function inv_k_vec