{at: 1, expect: "20% 30% 40px"},
        {at: 2, expect: "calc(-10px + 40%) calc(-20px + 60%) 50px"}
      ]);

      test_interpolation({
        property: 'grid-template-columns',
        from: "1fr 1fr 1fr",
        to: "2fr auto 2fr"
      }, [
        {at: -1, expect: "0fr 1fr 0fr"},
        {at: 0, expect: "1fr 1fr 1fr"},
        {at: 0.4, expect: "1.4fr 1fr 1.4fr"},
        {at: 0.6, expect: "1.6fr auto 1.6fr"},
        {at: 1, expect: "2fr auto 2fr"},
        {at: 2, expect: "3fr auto 3fr"}
      ])