compute x-y for x > y.  Get 0 if x < y.

Represent x-y by strings of 1's separated by -.  Mark front of string with 
#.  Move to the y string and repeat: mark the 1 at the front with a t, go
back to x and remove a 1 at the tail (write blank on it), until you run
out of 1's in y or until you hit #.  If you hit #, remove #, remove - and
remove all t's.  If you ran out of 1's, remove t's, remove # and -.

(0111l   State 1:  mark the leading #
(1 #2r

(2112r   State 2:  locate the y strin
(2  2r
(2--3r   State 3:  found the y string... mark a 1 with a t
(3tt3r
(31t4l   State 4:  marked a 1 in y.  Now go back to x string
(3  6l   State 6:  Ran out of 1's in y.  Clean up and halt
(4tt4l
(4--5l   State 5:  found x string.  Now remove any remaining 1's
(5  5l
(51 2r    remove a 1
(5# 7r   State 7:  more y's than x's.  Remove everything on the tape

(6t 6l
(6- 6l
(6  6l
(6116l
(6# 9r   State 9:  final state

(7  7r
(7- 8r
(8t 8r
(81 8r
(8  9r