how to auto purge automatically installed packages using aptitude?

[listofx]

Is there an equivalent command for aptitude which works like
$ apt-get --purge autoremove pkg

I have tried 
$ sudo aptitude --purge-unused remove xpdf

and found that aptitude just removed xpdf package only and had other
automatically installed packages left.

In other words, 
$ aptitude remove xpdf 
is different with 
$ aptitude --purge-unused remove xpdf. The former would remove related
packages which xpdf depends on. But the latter would not. I am not sure
the actual meaning of --purge-unused. Or there is a bug revealed.