gistfile1.txt

[1] pry(main)> h = Hash.new({})
=> {}
[2] pry(main)> h[:a]
=> {}
[3] pry(main)> h["a"]
=> {}
[4] pry(main)> h
=> {}
[5] pry(main)> h = Hash.new { |k, v| k[v] = {} }
=> {}
[6] pry(main)> h[:a]
=> {}
[7] pry(main)> h
=> {:a=>{}}