irb(main):001:0> h = Hash.new([])
=> {}
irb(main):002:0> h[:foo] << :bar
=> [:bar]
irb(main):003:0> h[:foo] << :baz
=> [:bar, :baz]
irb(main):004:0> p h
{}
=> nil
irb(main):005:0> h[:foo] = h[:foo]
=> [:bar, :baz]
irb(main):006:0> p h
{:foo=>[:bar, :baz]}
=> nil