class ActiveSupport::HashWithIndifferentAccess

counters.fetch(:zoo) # => KeyError: key not found: "zoo"
counters.fetch(:bar) {|key| 0} # => 0
counters.fetch(:bar, 0) # => 0
counters.fetch('foo') # => 1

counters[:foo] = 1
counters = ActiveSupport::HashWithIndifferentAccess.new

either a string or a symbol:
Same as Hash#fetch where the key passed as argument can be

def fetch(key, *extras)
  super(convert_key(key), *extras)
end