void AddPositionalParameter (object value)
{
if (positionalParameters == null) {
positionalParameters = new List<object> ();
if (Arguments.Count > 0)
{
// positional parameters can't come after non-positional parameters
throw Error("Unexpected positional parameter in expression '{0}'", this.value);
}
Arguments.Add (XamlLanguage.PositionalParameters, positionalParameters);
}
positionalParameters.Add (value);
}