* Iterate over key-value pairs of a map

    map = new('map 1:one 2:two 3:three')

visit(domain(map),'expr to evaluate for each member')

visit(range(map),'expr to evaluate for each member')

    next
        this = next(map)             :f(done)
        out(show(this) ':' show(get(map,this)) :next)
    done

    loop(d = domain(map)

    next
        out('next domain entry',next(d))    :s(next)
    done

    loop(d = range(map)

    next
        out('next domain entry',next(d))    :s(next)
    done