The structure is said to be implicit because it has not been declared explicitly as done when we specify the schema of a database.
The structure might vary from one to another, so it call that actual data is semi-structured.
The problem of extracting data from Web sources can be stated as follow: Given a Web page S containing a set of implicit objects, determine a mapping W that populates a data repository R with the object in S.
An important point connecting the generation of such a mapping is defining what an object is.
They present a new approach for generating wrappers that implement the mapping W, which is called DEByE.
Motivation: to let the user or database designer specify a target structure for the data to be extracted.
In DEByE, the examples of objects are used to generate Object Extraction Pattern (oe-patterns).These oe-patterns combine structural and textual information that are used to recognize and extract new objects.
Use the notion of an object type to represent complex object. An object of a specific object type is called an instance of the type.