I must have done this many times before
The answer to the puzzle is now clear
A wall of crystal blocks the exit door
I must have done this many times before.
Each futile blow I strike in mortal fear
Then dying, I reboot and live once more
I must have done this many times before
The answer to the puzzle is now clear.

A/N A triolet: I'm hoping that the repetition of lines in the poem reinforces the endless repetitions of the Doctor's actions in Heaven Sent. Don't know if it really works.