ruby idiom a | |= b meaning?

What does the ruby idiom a ||= b mean?

Thanks,
Ratnavel

It is like:
  a || a = b
See David A. Black's Blog
   http://dablog.rubypal.com/2008/3/25/a-short-circuit-edge-case

-Rob

Rob Biedenharn http://agileconsultingllc.com
Rob@AgileConsultingLLC.com