
hierlists.ss defines mred:hierlist^
hierlistr.ss returns a unit/sig, importes mred^ and exports mred:hierlist^
hierlist.ss invoke-opens hierlistr.ss using mred:... globals and the `mred' prefix

hierlist defines three classes:

 hierarchical-list%, derived from mred:media-canvas%
   
  New methods:
   (get-selected) - returns #f or an instance of  hierarchical-list-item%
   (new-item) - creates and returns a new (empty) hierarchical-list-item%
   (new-list) - creates and returns a new (empty) hierarchical-list-compound-item%
   (delete-item i) - delete hierarchical-list-item% i from the list
   (get-items) - returns a list of the items in the list
   (select i) - called for new select of i, hierarchical-list-item%  or #f,
                where #f means that no item is now selected
   (double-select i) - called for double-click on i hierarchical-list-item% i
   (item-opened i) - called when the arrow on i is turned down
   (item-closed i) - called when the arrow on i is turned up

 hierarchical-list-item%

   (get-buffer) - returns wx:media-edit% buffer containing the display representation 
                  of the item
   (is-selected?) - returns #t or #f
   (select on?) - selects or deselects the item; hierarchical-list%'s select is called
   (user-data) - returns user data; initialized to #f
   (user-data v) - sets user data to v

 hierarchical-list-compound-item%, derived from hierarchical-list-item%

   (new-item) - creates and returns a new (empty) hierarchical-list-item%
   (new-list) - creates and returns a new (empty) hierarchical-list-compound-item%
   (delete-item i) - delete hierarchical-list-item% i from the compound item
   (get-items) - returns a list of the items in the compound item
