+ <b>index.header.<prefix></b> <b>[STORED</b> <b>IN</b> <b>DATABASE]</b>
+ Define the query prefix <prefix>, based on a mail header. For
+ example <b>index.header.List=List-Id</b> will add a probabilistic pre‐
+ fix <b>List:</b> that searches the <b>List-Id</b> field. User defined pre‐
+ fixes must not start with 'a'...'z'; in particular adding a pre‐
+ fix with same name as a predefined prefix is not supported. See
+ <a href='../notmuch-search-terms-7/'>notmuch-search-terms</a>(7) for a list of existing prefixes, and an
+ explanation of probabilistic prefixes.
+