A search engine that is a participant in the help search. All engines selected by the user to run are executed as background jobs using the same search expression. Engine is expected to accept the expression and the scope object. The results should be passed to the result collector object, and the search progress should be tracked by the progress monitor.

Search engines that have API access to the search server can provide a list of individual search results. Search engines for which only the URL is known are expected to return one search result containing a URL that is sufficient to open a web browser and see the search results there.

Search engine must be cancelable.

This interface is intended to be implemented by clients.