irb(main):032:0> ratings = Hash.new(0)
=> {}
irb(main):033:0> books.values.each { |rate| ratings[rate]+= 1 }
=> [:super, :super, :nul, :archi_nul]
irb(main):034:0> ratings
=> {:super=>2, :nul=>1, :archi_nul=>1}
irb(main):035:0> ratings = {}
=> {}
irb(main):036:0> books.values.each { |rate| ratings[rate]+= 1 }
NoMethodError: undefined method `+' for nil:NilClass
        from (irb):36:in `block in irb_binding'
        from (irb):36:in `each'
        from (irb):36
        from /usr/bin/irb:12:in `<main>'