irb:11:26:09:023 > x = [ :a, :b, :c ].map { |key| { key => Hash.new([]) } }.reduce :merge
 => {:a=>{}, :b=>{}, :c=>{}}
irb:11:26:49:024 > x[:a]
 => {}
irb:11:26:54:025 > x[:a][:bloo] << :blah
 => [:blah]
irb:11:27:04:026 > x
 => {:a=>{}, :b=>{}, :c=>{}}
irb:11:27:06:027 > x[:a]
 => {}
irb:11:27:08:028 > x[:a][:bloo]
 => [:blah]