void on_sight( object ob, object source )

This is used for when an object appears in a room. This usually means the
object is a living, and it was walking around. The event might be added to
other things (such as dropping, crafting, building, etc.) but it isn't
currently. The source is where the object was before it moved (note: if
the object was previously concealed, such as might be with a thief's
sneaking abilities, it is possible for the source and the current location
to be the same).

Note that much of this is just potential use of this event -- at present
it is only used for living objects which are walking around.

FOR SCRIPTS: on_sight

Called when you see something. Actor is the thing you see. Generally this 
is reserved for people or animals entering a room; you can't use it for 
comprehensive surveys of everything visible.

