set_eq l1 l2 returns true if every element of l1 appears in l2 and
every element of l2 appears in l1. Otherwise it returns false. In other
words, it tests if the lists are the same considered as sets, i.e. ignoring
duplicates.
FAILURE CONDITIONS
Never fails.
EXAMPLE
# set_eq [1;2] [2;1;2];;
val it : bool = true
# set_eq [1;2] [1;3];;
val it : bool = false