The Riddle of the Time Machine

 

I bought a time-machine.

Killed Hitler.

Visited Atlantis and Pompeii.

Stopped my parents from marrying.

I came back to the present.

I am still there.

Customer service says, 

"Sorry. No refund."

This particular problem in fiction has been troubling me. As much as the doppelganger-should-die issue.

'Standard' fiction with time-machines usually has these rules: do not mess with your own past to kill your present; mess with your future to correct your present.

Somehow, I fear, the present will be preserved irrespective of what you do in the past or in the future. 

Let's call it the Riddle of the Time Machine.