This is the class of objects which may contain other objects, e.g., rooms, bags, creatures, etc.
#include <class.h>
inherit HOLDER;
void create() {
  holder::create();
  // initalizations from class object:
  set_name("name"); // required
  set_alias(({"alias1", ..., "aliasN"}));  // optional
  // initalizations from ability containable:
  set_mass(mass); // required (has alternative)
  set_bulk(bulk); // required (has alternative)
  // initializations from ability container:
  set_mass_capacity(mass capacity); // required
  set_bulk_capacity(bulk capacity); // required
  set_mass_factor(mass factor); // optional
  set_bulk_factor(bulk factor); // optional
  set_long_in("long inside description"); // required
}
void set_long_in(mixed s)
set_req_handler("/visual/long_in", s).
  See message types specific to classes
  for further explanation.
string query_long_in()
request_msg("/visual/long/in").