0 1 x r 0   ; Mark first number with x's.
0 _ s r 1   ; Mark space with s and start marking second number.
1 1 x r 1   ; Mark second number with x's.
1 _ s l 2   ; Start looking for digits to add.

2 x 1 r 3   ; Found digit to add -- now add.
2 _ _ r 4   ; Finished parsing all numbers -- clean up.
2 * * l 2   ; Ignore everything else.
3 _ z l 2   ; Add digit -- now look for more digits.
3 * * r 3   ; Ignore everything else.
4 s _ r 4   ; Replace s with _.
4 z 1 r 4   ; Replace z with 1.
4 _ _ l 5   ; Finished cleaning up -- now move to second-to-last number.
4 * * r 4   ; Ignore everything else.

5 1 1 l 5   ; Move past last number.
5 _ _ l 6   ; Begin moving past second-to-last number.
6 1 1 l 6   ; Move past second-to-last number.
6 _ _ r 0   ; Moved to correct location -- now produce next Fibonacci number.