void forceStackTop(Type t) {
            if (!alive) return;
            switch (t.tag) {
            case CLASS:
            case ARRAY:
                int width = width(t);
                Type old = stack[stacksize-width];
                Assert.check(types.isSubtype(types.erasure(old),
                                       types.erasure(t)));
                stack[stacksize-width] = t;
                break;
            default:
            }
        }