have nontrivial translation are the same aside from the last byte.
This makes it possible to translate just the last byte of a
character, and do so after just a simple test of the context.
- CHARSET_BASE is nonzero iff there is such a non-ASCII character.
+ CHARSET_BASE is nonzero if there is such a non-ASCII character.
If that criterion is not satisfied, do not call this function. */