imports system
namespace vbnetbook.chapter01
  public class stack
      dim top as entry
      public sub push(byval data as object)
         top=new entry(top,data)
      end sub
  public function pop() as object
      if(top is nothing) then
         throw new invalidoperationexception()
      end if
      dim result as object=top.data
      top=top.nextdata
      return result
   end function
   class entry
      public nextdata as stack.entry
      public data as object
            Sub New(ByVal nextdata As stack.entry, ByVal data As Object)
                Me.nextdata = nextdata
                Me.data = data
            End Sub
    end class
  end class
end namespace