The type or namespace name 'ISet' could not be found

Hi all. I want to use ISet<> in System.Collections.Generic namespace. I try to declare "using System.Collections.Generic" but it didn't work, although i can use IList<> and some interface or class in this namespace. So, how can i use ISet<>? (to mapping in NHibernate mapping)

P/S: I'm using .NET 3.5 SP1 and VS 2k8 Professional

Thanks