anchor.diff:      logical(c_bool), intent(inout) :: success
       ! Variables outside of signature.
       logical :: case_success
  -    real(c_double) :: nan_val
       real(c_double) :: start_node1(1, 2), end_node1(1, 2), error1
       real(c_double) :: start_node2(1, 2), end_node2(1, 2), error2
       real(c_double) :: nodes1(3, 2), nodes2(3, 2)
  
positive.code:     start_node2 = nodes2(:1, :)
      end_node2 = nodes2(3:, :)
      ! NOTE: This curve isn't close to linear, but that's OK.
      error2 = nan_val
  
      call from_linearized( &
           error1, 0.0_dp, 1.0_dp, start_node1, end_node1, 3, nodes1, &
  
negative.code: 
      case_id = 1
      name = "from_linearized"
  
      ! CASE 1: Basic test of not-very-linearized quadratics.
      nodes1(1, :) = 0
  