  Let G be a context-free grammar with a total alphabet V, and let F be a final
language over an alphabet W such that W is a subset of V. A final sentential
form is any sentential form of G that, after omitting symbols from V - W, it
belongs to F. The string resulting from the elimination of all nonterminals
from W in a final sentential form is in the language of G finalized by F if and
only if it contains only terminals.
  The language of any context-free grammar finalized by a regular language is
context-free. On the other hand, it is demonstrated that L is a recursively
enumerable language if and only if there exists a propagating context-free
grammar G such that L equals the language of G finalized by {w#w^R | w is a
string over a binary alphabet}, where w^R is the reversal of w.
