// sub.tm 
// proper subtraction  n-m  represented  0^n 1 0^m
//                     results in 0^(n-m)  for n >= m
//                     blank required at beginning and end
start q0
limit 100
halt q6

// delta transitions  p152  1)  through  6)
q0  #b q0  ## R
q0  0  q1  #b R   1)
q1  0  q1  0  R   2)
q1  1  q2  1  R
q2  1  q2  1  R   3)
q2  0  q3  1  L
q3  0  q3  0  L   4)
q3  1  q3  1  L
q3  #b q0  #b R
q2  #b q4  #b L   5)
q4  1  q4  #b L
q4  0  q4  0  L
q4  #b q6  0  R
q0  1  q5  #b R   6)
q5  0  q5  #b R
q5  1  q5  #b R
q5  #b q6  #b R
q6  #b q6 // final state

enddef

tape #b00000100#b  results in tape  000   i.e.  5-2=3
tape #b010#b       zero result, blank tape
tape #b0100#b      bad, negative, zero blank tape
tape #b000010#b    result 000
tape #b00001000#b  result 0