This TM accepts strings w where n_a(w) = n_b(w).  Namely, strings where
the number of a's is equal to the number of b's

(0ax1l  State 0:  find an 'a' and mark it. Start from front of string
(0bb0r
(0xx0r

(1bb1l  State 1:  Had found an 'a'. Go back to start of string and look for 'b'
(1xx1l
(1  2R

(2aa2r  State 2:  Look for a 'b' and mark it.
(2xx2r
(2bx3l   

(3xx3l  State 3:  Have matched a pair of a's and b's.  Go back and repeat.
(3aa3l
(3bb3l
(3  0R

(0  4l  State 4:  Ran out of a's.  Go back to see if there are no more b's.
(4xx4l
(4  5l  State 5:  Final state.  The string is all x's.