.. efun:: string program_name()
  string program_name(object obj)

  Returns the name of the program of :arg:`obj`, resp. the name of the program of the current object if :arg:`obj` is omitted.

  The returned name is usually the name from which the blueprint of :arg:`obj` was compiled (the 'load name'), but changes if an object replaces its programs with the :efun:`replace_program`.

  As a special case, if :arg:`obj` is passed as 0, the function will return 0.

  The name always ends in '.c'. It starts with a '/' unless the driver is running in :macro:`__COMPAT_MODE__`.

  .. warning:: This efun swaps in the program if it is swapped out.

  .. usage::

    ::

      object o;
      o = clone_object("/std/thing");
      write(program_name(o)); // writes "/std/thing.c" in !compat mode
                              // and "std/thing.c"  in compat mode

  :history 3.2.6 introduced:
  :history 3.2.9 changed: allowed a 0 argument.

  .. seealso:: :efun:`clone_object`, :efun:`clonep`, :efun:`load_name`, :efun:`load_object`, :efun:`object_name`, :efun:`replace_program`