> Why the difference doesn't work like [1, 2] - [2] ?
> --

Prova.new(2) and Prova.new(2) are two different objects.  But with
Fixnums, there is only ever one 2.

?> p1 = [Prova.new(1), Prova.new(2)]
=> [#<Prova:0x2e91c34 @n=1>, #<Prova:0x2e91c20 @n=2>]
>> p2 = [Prova.new(2)]
=> [#<Prova:0x2e8fcf4 @n=2>]
>> puts (p1 - p2)
1
2
=> nil
>>