Sl rib

I am knitting a sweater and the directions say to slip rib. I have looked in my Knitting book but I can’t find any info. on it. Does anyone know what slip rib means?

Could it be the sock heel stitch?

Hopefully someone who owns ‘Knitting on the Edge’ by Nicky Epstein will look at this post. I have borrowed it from the library before and I am fairly certain that the slip rib is in there but I don’t have the book right now.