The example above is wrong though.
The documentation on the wiki is correct. Sparse, but correct.
Each event returns some event specific parameters.
The
event handler function returns an event code parameter PLUS the parameters from the specified event.
It's like asking what sandwich filling you want. The bread will always be there, as it wouldn't be a sandwich without the bread, but you don't need to specify bread because we all know the sandwich starts with bread.
Yes, I'm aware there are different types of bread, but that's not the point.
