For example:

@interface Fraction: NSObject {
...

When wouldn't NSObject be used and is NSObject the ultimate parent class for all other classes?

Please feel free to correct me on any wrong terminology used.

Does anyone know how to delete all datastore in Google App Engine?

Does anyone know ?

when using mvvm pattern, when we have lists we use ObservableCollection<>.

my question is when should i call to

CollectionViewSource.GetDefualtView(theCollection);

to get the view that i can filter and sort and do some other cool stuff.

if i dont work with mvvm i’ll do it in window.load

I am adding subscriptions to a site using Paypal IPN which works very well, I can successfully create a new subscription and verify it. The subscription has a two week free trial. The guide was unfortunately a little vague on subscription statuses.

At the moment, the users account gets subscribed status once subscr_signup or subscr_payment is received and gets removed when either subscr_cancel or subscr_failed is received. I believe this is correct but it's best to make sure.

Also what is subscr_eot? the IPN guide describes it as "subscription’s end-of-term." Does this get triggered after the trial period is over?

In Latex, how do I eliminate the space inserted before itemize?

\begin{itemize} % produces lots of vertical space
\item ...
\item ...
\end{itemize}