Initializing History and State Management
-----------------------------------------

   This section describes functions used to initialize and manage the
state of the History library when you want to use the history functions
in your program.

 - Function: void using_history ()
     Begin a session in which the history functions might be used.  This
     initializes the interactive variables.

 - Function: HISTORY_STATE * history_get_history_state ()
     Return a structure describing the current state of the input
     history.

 - Function: void history_set_history_state (HISTORY_STATE *state)
     Set the state of the history list according to STATE.