public void MathExpressionContext_Cannot_Define_Variable_That_Explicitly_Refers_To_Itself()
{
MathExpressionContext context = new MathExpressionContext();
var exception = Assert.Throws<MathExpressionException>(() => context.Define("x", MathExpression.Parse("x + 5")));
Assert.AreEqual("Variable 'x' is refering to itself recursively.", exception.Message);
}