This routine adds the appearance object to the Library. (If the appearance passed in is
None, this appearance is will not be added to the Library.) There is no checking for duplicates when adding an appearance. This call will always add the appearance to the Library, even if an identical appearance already exists therein. If the addition is successful, the method returns a unique appearance reference ‘key’ that is associated with the appearance. If there are any errors in operation, an exception is raised. If the appearance is successfully added to the library, the library consumes the appearance.