Not sure if this is an intentional restriction but the following doesn't seem to work

.mixin() {
    a {
        color: red;
        span { color: blue; }
    }
}
.class { .mixin; }

should output

.class a {
    color: red;
}
.class a span {
    color: blue;
}

it only works with one level of nesting, the second level is lost when you run this code in mixin.js:

return new(tree.Ruleset)(rule.selectors, rule.rules.map(function (r) {
    return new(tree.Rule)(r.name, r.value.eval(context));
}));

You could also change this area of code to allow a mixin.Call or even a mixin.Definition inside another mixin.Definition.