What does backslash mean in SPIN?

I sometimes see code like
if i := \bt.Start
What does the backslash used for?
I searched the SPIN Quick Reference but came up with nothing.
if i := \bt.Start
What does the backslash used for?
I searched the SPIN Quick Reference but came up with nothing.
Comments
It's "abort trap".
Please check P47 and P208 on Propeller Manual v1.1.