To the left of M is a sequence of one or more left labels L i (c) including the special termination symbol , which indicates that there are no more symbols to the left, and similarly for the labels to the right, R i (c).
an expansion e(c) looks like: The expansion is generated by guessing first M, then in order L 1 through L m+1 (= ), and similarly for R 1 through R n+1 .