Used by the routines at TOO_DARK and IN_REACH_OF.

Follows byte 1 of the object's record -- what holds it -- up through holders whose flags have $28 set, which can be seen into, and returns the first that cannot, or $FF if it runs out. The player's own record has $28 set, so a thing the player carries is not shut in anything by being carried.

Byte 1 is watched as well as read: the map is held by Gandalf ($3E) on the tape and by the player (0) at the first prompt, the turn the game says Gandalf gives it to you; and the large key is held by the hideous troll, as the trolls' clearing says it is.