public PackageSymbol enterPackage(Name fullname) {
        PackageSymbol p = packages.get(fullname);
        if (p == null) {
            Assert.check(!fullname.isEmpty(), "rootPackage missing!");
            p = new PackageSymbol(
                Convert.shortName(fullname),
                enterPackage(Convert.packagePart(fullname)));
            p.completer = this;
            packages.put(fullname, p);
        }
        return p;
    }