mergeHandler( $diff, $old, $new, \%info ) -> $text

• $diff => one of the characters '+', '-', 'c' or ' '.
  • '+' - new contains text inserted in the new version
  • '-' - old contains text deleted from the old version
  • 'c' - old contains text from the old version, and new text from the version being saved
  • ' ' - new contains text common to both versions, or the change only involved whitespace
• $old => text from version currently saved
• $new => text from version being saved
• \%info is a reference to the form field description { name, title, type, size, value, tooltip, attributes, referenced }. It must not be wrtten to. This parameter will be undef when merging the body text of the topic.

Plugins should try to resolve differences and return the merged text. For example, a radio button field where we have { diff=>'c', old=>'Leafy', new=>'Barky' } might be resolved as 'Treelike'. If the plugin cannot resolve a difference it should return undef.

The merge handler will be called several times during a save; once for each difference that needs resolution.

If any merges are left unresolved after all plugins have been given a chance to intercede, the following algorithm is used to decide how to merge the data:

1. new is taken for all radio, checkbox and select fields to resolve 'c' conflicts
2. '+' and '-' text is always included in the the body text and text fields
3. <del>conflict</del> <ins>markers</ins> are used to mark 'c' merges in text fields

The merge handler is called whenever a topic is saved, and a merge is required to resolve concurrent edits on a topic.