What's the function of `<T>` in `fn foo<T>() = default`? : In `fn foo<T>() = default;` what does the `<T>` actually do and refer to? Can I omit it when my Event doesn't include for example `AccountId`? 

This is referring to http://shawntabrizi.com/substrate-collectables-workshop/#/2/creating-an-event?id=depositing-an-event.