I recently had to change the password on my work computer. I keyed in "Sum(n)=n/2(A1+An)".
The system gave me a message that my proposed new password "is not unique enough."
Unique is a binary choice. Something is either unique or it's not. And what was the problem with my prospective new password? I tried several other entries, with the same response.
So I called our help desk. The helpful help desk guy explained that I needed to make my password uniquer.
Any system that won't take THAT as a password needs to be reprogrammed!