History Storage
===============

   The history list is an array of history entries.  A history entry is
declared as follows:

     typedef struct _hist_entry {
       char *line;
       char *data;
     } HIST_ENTRY;

   The history list itself might therefore be declared as

     HIST_ENTRY **the_history_list;

   The state of the History library is encapsulated into a single
structure:

     /* A structure used to pass the current state of the history stuff around. */
     typedef struct _hist_state {
       HIST_ENTRY **entries;         /* Pointer to the entries themselves. */
       int offset;                   /* The location pointer within this array. */
       int length;                   /* Number of elements within this array. */
       int size;                     /* Number of slots allocated to this array. */
       int flags;
     } HISTORY_STATE;

   If the flags member includes `HS_STIFLED', the history has been
stifled.