Problem 4.a from Skvarcius and Robinson.  Find a TM that recognizes 
{a^nb^n | n>=0} that works from the inside out.  See page 391.
(0aa0R   move past the inital a's in the string.  
(0AA0R   move past old a's
(0BB0R        and old b's
(0bB1L   state 1: found a new b, changed it to B, go back for matching a
(0  2L   no more b's left.  Check that all a's were marked.
(1BB1L   
(1AA1L
(1aA0R   found a new a.  change it to an A, look for another b
(2AA2L
(2BB2L
(2aa3L   found an a.  halt, non-final state
(3**3L   
(2  9R)  state 9: final state.  all b's were changed to B's
(1  3L