; Given a string in {a,b}*, rewrite it as the next
; string in this enumeration:
;   empty, a, b, aa, ba, ab, bb, aaa, baa, aba, bba, aab, ...
; This TM never halts, but it pauses to let us see each string.

Repeat forever (but pausing for each string).

0 b a r 0
0 a b l 1
0 _ a l 1
1 a a l 1
1 _ _ r 0 ! ; pause with "!" to see current string