Patch for backwardSlice.

Hi,
I think there's a bug in backwardSlice: The sideEffect { first=False } should be executed once all direct neighbors in the graph are processed. Right now, it is processed after one of them has been processed.
I think this patches the issue by gathering neighbors after each execution of the loop and then executing the side-effect. If this is correct, you probably also want to make these changes to forwardSlice.
Fabian