Information About the History List
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   These functions return information about the entire history list or
individual list entries.

 - Function: HIST_ENTRY ** history_list ()
     Return a `NULL' terminated array of `HIST_ENTRY' which is the
     current input history.  Element 0 of this list is the beginning of
     time.  If there is no history, return `NULL'.

 - Function: int where_history ()
     Returns the offset of the current history element.

 - Function: HIST_ENTRY * current_history ()
     Return the history entry at the current position, as determined by
     `where_history ()'.  If there is no entry there, return a `NULL'
     pointer.

 - Function: HIST_ENTRY * history_get (int offset)
     Return the history entry at position OFFSET, starting from
     `history_base'.  If there is no entry there, or if OFFSET is
     greater than the history length, return a `NULL' pointer.

 - Function: int history_total_bytes ()
     Return the number of bytes that the primary history entries are
     using.  This function returns the sum of the lengths of all the
     lines in the history.