anchor.diff:    subroutine helpers_all_tests(success)
       logical(c_bool), intent(inout) :: success
   
  +    call test_cross_product(success)
       call test_vector_close(success)
       call test_in_interval(success)
   
     end subroutine helpers_all_tests
  +
  +  subroutine test_cross_product(success)
  +    logical(c_bool), intent(inout) :: success
  +    ! Variables outside of signature.
  +    real(c_double) :: vec0(1, 2)
  +    real(c_double) :: vec1(1, 2)
  +    real(c_double) :: result_
  +
  +    ! CASE 1: Identical vector.
  +    vec0(1, :) = [1.0_dp, 7.0_dp] / 8
  +    vec1(1, :) = [-11.0_dp, 24.0_dp] / 32
  +    call cross_product(vec0, vec1, result_)
  +    if (result_ == 101.0_dp / 256) then
  +       write (*, "(A)") "cross_product: Case 1 success"
  +    else
  +       write (*, "(A)") "cross_product: Case 1 failure"
  +       success = .FALSE.
  +    end if
  +
  +  end subroutine test_cross_product
   
     subroutine test_vector_close(success)
       logical(c_bool), intent(inout) :: success
  
positive.code:     vec2(1, :) = 0.5_dp**45 * [3.0_dp, 4.0_dp]
      is_close = vector_close(2, vec1, vec2, eps)
      if (is_close) then
         write (*, "(A)") "vector_close: Case 5 success"
      else
         write (*, "(A)") "vector_close: Case 5 failure"
         success = .FALSE.
      end if
  
  
negative.code:     ! CASE 1: Interior value.
      is_inside = in_interval(1.5_dp, 1.0_dp, 2.0_dp)
      if (is_inside) then
         write (*, "(A)") "  in_interval: Case 1 success"
      else
         write (*, "(A)") "  in_interval: Case 1 failure"
         success = .FALSE.
      end if
  
      ! CASE 2: Barely inside.
      is_inside = in_interval(1.0_dp + 0.5_dp**52, 1.0_dp, 2.0_dp)
      if (is_inside) then
         write (*, "(A)") "  in_interval: Case 2 success"
      else
         write (*, "(A)") "  in_interval: Case 2 failure"
         success = .FALSE.
      end if
  
      ! CASE 3: Barely outside.
      is_inside = in_interval(1.0_dp - 0.5_dp**53, 1.0_dp, 2.0_dp)
      if (.NOT. is_inside) then
         write (*, "(A)") "  in_interval: Case 3 success"
      else
         write (*, "(A)") "  in_interval: Case 3 failure"
         success = .FALSE.
      end if
  
      ! CASE 4: Exterior value.
      is_inside = in_interval(-1.0_dp, 1.0_dp, 2.0_dp)
      if (.NOT. is_inside) then
         write (*, "(A)") "  in_interval: Case 4 success"
      else
         write (*, "(A)") "  in_interval: Case 4 failure"
         success = .FALSE.
      end if
  
  