See 
PublishedAPI for packages intended to be used by Plugin and Contrib authors, or 
browse all packages.
See also 
Developing plugins, 
Developer's Bible, 
Technical Overview
Base class of certificate checkers.
This checker validates files that contain X.509 certificates,
such as for the S/MIME signatures and for the Tasks framework.
It must be subclassed for the various certificate types, as the requirements
are slightly different.
 ObjectMethod checkUsage( $keys, $usage, $reporter ) → @subjects 
Validates a Certificate item 
-  $keysitem to check
-  $usage- Required use (email, client, server, clientserver)
Returns list of subjects.