*  @param other  The overridden method.
     *  @return       An internationalized string.
     */
    Object cannotOverride(MethodSymbol m, MethodSymbol other) {
        String key;
        if ((other.owner.flags() & INTERFACE) == 0)
            key = "cant.override";
        else if ((m.owner.flags() & INTERFACE) == 0)
            key = "cant.implement";
        else
            key = "clashes.with";
        return diags.fragment(key, m, m.location(), other, other.location());
    }