s method within a serializable transaction will acquire a
   nonblocking predicate lock on the full index.  This will generate a
   read-write conflict with the insert of any tuple into that index by a
   concurrent serializable transaction.  If certain patterns of read-write
   conflicts are detected among a set of concurrent serializable
   transactions, one of those transactions may be canceled to protect data
   integrity.  When the flag is set, it indicates that the index access
   method implements fi