public void ParseSimpleSubtract()
{
Parser parser = new Parser("10-20.");
IExpression expression = parser.ParseExpression();
Assert.IsNotNull(expression);
Assert.IsInstanceOfType(expression, typeof(SubtractExpression));
SubtractExpression subtractexpression = (SubtractExpression)expression;
Assert.IsInstanceOfType(subtractexpression.LeftExpression, typeof(ConstantExpression));
Assert.IsInstanceOfType(subtractexpression.RightExpression, typeof(ConstantExpression));
Assert.AreEqual(10, ((ConstantExpression)subtractexpression.LeftExpression).Value);
Assert.AreEqual(20, ((ConstantExpression)subtractexpression.RightExpression).Value);
}