Having a class in a package, when you try to move that class to another package, the following error is thrown:

[exemplo] model $ ls
Cidade.java               UnidadeFederativa.java    
[exemplo] model $ mv Cidade.java ex01/Cidade.java
***ERROR*** [mv] no such child: null

Probably we should give a better description, or say that this feature isn't supported yet.

Moving a class to another package includes refactoring the whole project, in order to avoid compilation errors.