consists of a sequence of cells with a number of 1's and the rest of the tape
blank and the reading head over the left most non-blank cell.
Suppose the reading head of TM M moves a maximum of X cells to the left of
the initial position. Then consider a TM M' that moves the block of 1's X+1
cells to the right (show this need never move to the left of the initial
position), then positions the reading head over the leftmost non blank cell
and then executes M.