I'm making a scarf with a twisted cable pattern (lace) and the abbreviation yrn shows up a few times. I would think it means "yarn" but it makes no sense to me. It either says yrn, P4 or it says yrn P3. A couple of times it just says yrn. I've not done lace before, so is this something unique to lace, unique to this pattern, or unique to cables?