gistfile1.rb

irb(main):001:0> a, b = nil, 2
=> [nil, 2]
irb(main):002:0> b ||= a
=> 2
irb(main):003:0> b
=> 2
irb(main):004:0> a
=> nil
irb(main):005:0> a ||= b
=> 2
irb(main):006:0> b
=> 2
irb(main):007:0> a
=> 2