The type or namespace name 'ISet' does not exist

I'm upgrading a Windows Smart Client solution from nHibernate 2.2 to 4.0. This required updating Iesi.Collections (4.0.1.4000) as well.

On this line:

protected Iesi.Collections.Generic.ISet<PermitEvent> eventSet;

I get this error:

The type or namespace name 'ISet' does not exist in the namespace 'Iesi.Collections.Generic' 

This question has gone unanswered at 2 other sites:
http://www.c-sharpcorner.com/forums/the-type-or-namespace-name-iset-could-not-be-found http://www.resolvinghere.com/cc/the-type-or-namespace-name-iset-could-not-be-found.shtml

What can I do?