window every n elements instead of time based

[Adrian Mocanu]

Hi
Is there a way to do window processing but not based on time but every 6 items 
going through the stream?

Example:
Window of size 3 with 1 item "duration"
Stream data: 1,2,3,4,5,6,7
[1,2,3]=window 1
[2,3,4]=window 2
[3,4,5]=window 2
etc

-Adrian