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See also 
Developing plugins, 
Developer's Bible, 
Technical Overview
A cache of Meta objects - initially used to speed up searching and sorting, but by foswiki 2.0 hopefully this 
will be used for all readonly accesses to the store.
Replaces the mishmash of the Search 
InfoCache Support package; cache of topic info. 
When information about search hits is
compiled for output, this cache is used to avoid recovering the same info
about the same topic more than once.
 ClassMethod new($session) 
 ObjectMethod finish() 
Break circular references.
 Note to developers; please undef 
all fields in the object explicitly,
 whether they are references or not. That way this method is "golden
 documentation" of the live fields in the object.
 ObjectMethod hasCached($webtopic) → boolean 
returns true if the topic is already int he cache.
 ObjectMethod get($web, $topic, $meta) → a cache obj (sorry, needs to be refactored out to return a Foswiki::Meta obj only 
get a requested meta object - web or topic typically, might work for attachments too
optionally the $meta parameter can be used to add that to the cache - useful if you've already loaded and parsed the topic.
and then, the 
MetaCache can only return Meta objects that actually exist