See 
PublishedAPI for packages intended to be used by Plugin and Contrib authors, or 
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See also 
Developing plugins, 
Developer's Bible, 
Technical Overview
  internal package Foswiki::Contrib::PlainFileStoreContrib  
  bootstrapStore 
Class method called from configuration bootstrap to determine if this
store can be used.  If it's usable, it will take precedence over the RCS
based configurations.
This method "guesses" the following configuration settings
  
It must run after the 
DataDir and 
PubDir settings have been applied.
If it detects both RCS and 
PlainFile store files, it dies to prevent
history corruption.