Perforce触发器拒绝提交未更改的文件？

Perforce allows people to check in unchanged files. Why any version control system would allow this is beyond me, but that's a topic for another question. I want to create a trigger that will deny the submission of unchanged files. However, I have no experience with Perforce triggers. From what I've read, I'm guessing this would be a "Change-content" trigger since the files being submitted would have to be diffed against the respective head revisions they are about to replace. I would need to iterate over the incoming files and make sure they had all indeed changed. The problem is, I have no idea how to go about it.Perforce allows people to check in unchanged fi