In chapter Domain Object Security (ACLs) Spring Security 3.1 documentation, ACL is described. In the second paragraph, there is a pet clinic example:
To make it a little more interesting, your customers can allow other users to see their customer records
and in the following paragraphs different solutions, domain models, ACL tables, etc are described.

What I am looking for is a reference implementation where the ACL solution has been implemented that I can check out and play with. Do you if there is an example project and where it can be found?

Thanks in advance.