What can _not_ be marked pure?

[Guillaume Piolat via Digitalmars-d-learn]

If I understand purity correctly 
(http://klickverbot.at/blog/2012/05/purity-in-d/), every function 
out there can be marked pure as long as it doesn't modify 
globals, shared variables or do I/O?

It seems more function can be marked pure that I previously 
thought.