`delete_history'
----------------

delete_history([INDEX])
     :: Deletes the history.

RETURN
     0

INDEX
     Index of history to be deleted.

   * Deletes all the histories without an argument.

   * Deletes the history with index INDEX if specified.

   * A history is an expression which has been obtained by evaluating
     an input given for a prompt with an index. It can be taken out by
     `@INDEX', which means that the expression survives garbage
     collections.

   * A large history may do harm in the subsequent memory management and
     deleting the history by `delete_history()', after saving it in a
     file by `bsave()', is often effective.

     [0] (x+y+z)^100$
     [1] @0;
     ...
     [2] delete_history(0);
     [3] @0;
     0

