Mask Modifier: Invert button missing?

Support Basics & Interface
1

I’m messing around with the Mask Modifier. I apply a vertex group but I’m looking for the invert button. It appears in tutorials but I can’t find it in 2.73

thanks in advance!

2

OOPS. solved. Mods please delete.
facepalm
it’s the double arrow icon =P

3

Double arrow button