Matz:
> I'm thinking of another way:
> 
>      Hash.new{[]}
> 
> This means if the key is not found in the hash, evaluate given block
> and returns its value as default.  In this example, the block creates
> new array for each execution.

Sounds even better (more powerful) as one can easily have logic for
determination of what should be the default value this time.

	- Aleksi