I did a little playing with it last night.

I find that one can do object-oriented things, but constructors are a bit of a catch.

    struct A : [a, defaulted]
    struct B : A [b]
    
    //You can do it this way,
    //but each subclass has to init every field (brittle)
    function MakeA(a): [a, 99] : A
    function MakeB(a,b): [a, 99, b] : B
    
    //Or use the super constructor syntax
    //but this uses a copy and is less efficient
    function MakeA(a): [a, 99] : A
    function MakeB(a,b): [super MakeA(a), b] : B
    
    //Or something in between
    function InitA(me::A, theA): a = theA; defaulted = 99; me
    function MakeA(a): new := [0,0]; new.InitA(a)
    function InitB(me::B, theA, theB): me.InitA(theA); b = theB; me
    function MakeB(a,b): new := [0,0,0]; new.InitB(a,b)

I'll keep playing with it and see what else I discover. I haven't gotten far enough to use coroutines, multi-method, or to make my own control structures yet.