the woman shake her fists at Jasper in church, and hears from her
that she knows Jasper "better far than all the reverend parsons put
together know him." Datchery then adds a long thick line to his
chalked score, yet, says Mr. Walters, Datchery has learned "nothing
new to Edwin Drood, if alive."

This is an obvious error. It is absolutely new to Edwin Drood that
the opium hag is intimately acquainted with his uncle, Jasper, and
hates Jasper with a deadly hatred. All this is not only new to
Drood, if alive, but is rich in promise of further revelations.
Drood, on Christmas Eve, had learned from the hag only that she
took opium, and that she had come from town to Cloisterham, and had
"hunted for a needle in a bottle of hay." That was the sum of his
information. Now he learns that the woman knows, tracks, has
found, and hates, his worthy uncle, Jasper. He may well,
therefore, add a heavy mark to his score.

We must also ask, How could Helena, fresh from Ceylon, know "the
old tavern way of keeping scores? Illegible except to the scorer.
The scorer not committed, the scored debited with what is against
him," as Datchery observes. An Eurasian girl of twenty, new to
England, would not argue thus with herself: she would probably
know nothing of English tavern scores. We do not hear that Helena
ever opened a book: we do know that education had been denied to
her. What acquaintance could she have with old English tavern
customs?

If Drood is Datchery, then Dickens used a form of a very old and
favourite ficelle of his: the watching of a villain by an
improbable and unsuspected person, in this case thought to be dead.