/// <summary>
/// Called for ValueExpression and all subclasses
/// </summary>
/// <param name="expression"></param>
public virtual void OnValue(ValueExpression expression)
{
}
/// <summary>
/// Visit a value expression. NOTE: this method is not called for subclasses
/// of ValueExpression. To perform an operation on ValueExpression and all of its
/// subclasses, override the OnValue method.
/// </summary>
/// <param name="expression">value expression</param>
/// <seealso cref="OnValue"/>
public virtual void Visit(ValueExpression expression)
{
OnValue(expression);
}