irb(main):001:0> r = {}
=> {}
irb(main):002:0> r.default=[]
=> []
irb(main):003:0> r[1]
=> []
irb(main):004:0> r[1]  << 2
=> [2]
irb(main):005:0> r[2]
=> [2]
irb(main):006:0> r[3]
=> [2]
irb(main):007:0>