Equality is actually a rather complicated thing. The way I understand it 'is' is an object equality predicate, roughly equivalent to Scheme's 'eqv?' for most data types. The 'iso' function on the other hand sounds like it was intended to be a structural equivalence primitive (which is what isomorphism means in mathematics), which should roughly have the semantics of Scheme's 'equal?'. Arc 3.1 defined iso only for lists, but it seems that just about every other third-party implementation including my own Arcueid extended it to arbitrary other objects such as tables.
