Еще одна неожиданная упячка в ruby:

vitaly@way:~/projects$ irb
1.9.3p0 :001 > a
NameError: undefined local variable or method `a' for main:Object
from (irb):1
from /home/vitaly/.rvm/rubies/ruby-1.9.3-p0-perf/bin/irb:16:in `<main>'
1.9.3p0 :002 > a = a
=> nil
1.9.3p0 :003 > a
=> nil

1.9.3p0 :004 > a = b
NameError: undefined local variable or method `b’ for main:Object
from (irb):4
from /home/vitaly/.rvm/rubies/ruby-1.9.3-p0-perf/bin/irb:16:in `<main>’

Wtf?