*** Warning: the following definition does not cover 23 tuples in the relation NAME(A) := AllCaps(A), followed by(A,B)
NAME(A) := Upper(A), followed by(A,B), Number(B)
NAME(A) := followed by(A,B), Venice(B) STREET(A) := Large(A), followed by(A,B)
STREET(A) := Medium(A), followed by(A,B), AlphaNum(B)
** Warning: the following definition does not cover 9 tuples in the relation CITY(A) := Los(A) CITY(A) := Marina(A) CITY(A) := New(A)
CITY(A) := Brooklyn(A)
CITY(A) := West(A), followed by(A,B), Alpha(B)
STATE(A) := CA(A)
STATE(A) := NY(A)
PHONE(A) := ((A)
Table 10: Definitions learned by foil.6 for the Bigbook **source**
Rapture **decides that the wrapper is correct; otherwise, that it has failed. Kushmerick** found that the HTML density alone can correctly identify almost all of the changes in the sources he monitored. In fact, adding other features in the probability calculation significantly reduced algorithm's performance. We compared Rapture**'s performance on**
the verification task to our approach, and found that Rapture **missed 17 wrapper changes**
(false negatives) if it relied solely on the HTML density feature. 10 There has been relatively little prior work on the wrapper reinduction problem. Cohen (Cohen, 1999) adapted WHIRL**, a "soft" logic that incorporates a notion of statistical**
text similarity, to recognize page structure of a narrow class of pages: those containing simple lists and simple hotlists (defined as anchor-URL pairs). Previously extracted data, combined with page structure recognition heuristics, was used to reconstruct the wrapper once the page structure changed. Cohen conducted wrapper maintenance experiments using original data and corrupted data as examples for WHIRL**. However, his procedure for**
corrupting data was neither realistic nor representative of how data on the Web changes.

Although we cannot at present guarantee good performance of our algorithm on the wrapper reinduction for sources containing lists, we handle the **realistic data changes in Web**
sources returning detail pages.