Upsy.... It's fixed now. Will be up for the next version

Mmmm, really is not a bug. The only public constructor belongs to the private neasted classes inside the Immutable Data Structures. So the constructor is public only to the parent class, but not to the client code :)

public class ImmutableStack:IEnumerable
{
    private class ImmutableFullStack : ImmutableStack
    {
        readonly T head;
        readonly ImmutableStack tail;

        public ImmutableFullStack(T head, ImmutableStack tail)
        {
            this.head = head;
            this.tail = tail;
        }

     [...]
  }
 [...]

}