Visible()
FUNCTION: 
visible(<object>,<victim>[/<attr>])
Returns 1 if <object> can examine <victim>, or 0 otherwise. If <object> or <victim> do not exist, 0 is returned.
If an object-attribute pair is provided, 1 is returned if <object> can see <attr> on <victim>. If <attr> is not present on <victim>, 1 is returned if <object> can examine <victim>.