def extract(self, document, selector):
raise NotImplementedError<EOL>
Apply the selector on the document
and return a value for the matching elements (text content or
element attributes)

:param document: lxml-parsed document
:param selector: input :class:`.Selector`  to apply on the document
:rtype: depends on the selector (string, boolean value, ...)

Return value can be single- or multi-valued.