-If the current history offset refers to a valid history entry,
-increment the current history offset.
-If the possibly-incremented history offset refers to a valid history
-entry, return a pointer to that entry;
-otherwise, return a @code{BNULL} pointer.
+Move the current history offset forward to the next history entry, and
+return the a pointer to that entry. If there is no next entry, return
+a @code{NULL} pointer.