public void ParseCharacter()
{
ModelParser parser = new ModelParser("$a");
IExpression expression = parser.ParseExpression();
Assert.IsNotNull(expression);
Assert.IsInstanceOfType(expression, typeof(ConstantExpression));
ConstantExpression cexpression = (ConstantExpression)expression;
Assert.AreEqual('a', cexpression.Value);
Assert.AreEqual("$a", expression.AsString());
}