anchor.diff:  
     use iso_c_binding, only: c_double, c_bool
     use helpers, only: &
  -       WIGGLE, cross_product, bbox, wiggle_interval, vector_close, &
  -       in_interval
  +       WIGGLE, cross_product, bbox, wiggle_interval, contains_nd, &
  +       vector_close, in_interval, ulps_away
     use types, only: dp
     implicit none
     private test_cross_product, test_vector_close, test_in_interval
  
positive.code: 
    end subroutine test_wiggle_interval
  
    subroutine test_vector_close(success)
      logical(c_bool), intent(inout) :: success
      ! Variables outside of signature.
      logical(c_bool) :: is_close
      real(c_double) :: eps
      real(c_double) :: vec1(1, 2)
      real(c_double) :: vec2(1, 2)
  
      eps = 0.5_dp**40
  
  
negative.code:     call test_cross_product(success)
      call test_bbox(success)
      call test_wiggle_interval(success)
      call test_contains_nd(success)
      call test_vector_close(success)
      call test_in_interval(success)
      call test_ulps_away(success)
  
    end subroutine helpers_all_tests
  
  