4.
- if a substring on track 2 starting at the selected position doesnot match ,
the LBA halts in a rejecting state. - If the substring on track 2 starting at the selected position is but the string obtained by replacing by
(i.e. applying the rule 
) has a length greater than |w| , then the LBA halts in rejecting state. - otherwise, is replaced by on track 2.
5. If track 2 contains the string w, then the LBA halts in an accepting state, otherwise, steps 3 through 5 are repeated.
Thus, the LBA M will accpet a string w if 
. Conversely, a computation of the LBA M with input <w> that falls in an accepting state consists of a sequence of string transformations generated by steps 3 and 4. But these transformations define a deviation of w in G. Thus, the LBA M accepts w iff .