public void TryPoly3()
{
Entity expr = "x * (x + x2 + z) + y * x";
if (MathS.TryPolynomial(expr, x, out var dst))
{
Assert.Equal(MathS.FromString("x3 + x2 + (z + y) * x"), dst);
}
else
{
throw new Xunit.Sdk.XunitException($"{expr.Stringize()} is polynomial");
}
}