Ruby and CQRS - Command methods should return nothing

CQRS stands for Command Query Responsibility Segregation and the basic rules are:
Commands - Perform (side effects) actions on the system and don't return values.
Queries  - Return values but can't touch the system's state.
Idealy, you don't mix them. Hence you can say, commands are dangerous, and
queries are safe.

Now, ruby doesn't have a specific type to say: this method (command) returns nothing, like void in C and other languages. So, the question is: How do you state that a particular method returns nothing?

Based on comments from the ruby mailing list, people are using these approaches;
1. Return self.
2. Return nil.
3. Implicit return (which in general, ends up being nil).

If I'd have to pick one, I'd go with the last one, but I also like to throw
another option into the mix....
Why don't create a class for this particular use case? Let's say, Nothing!

module Nothing
class Nothing
end

def nothing
@nothing ||= Nothing.new
end
end

class CommandQueryApi
include Nothing

# By using the nothing helper, is clear that the method returns, nothing!
def cmd_foo
# Do something
nothing
end
end


I 've used this approach a couple of times, and at least to me and my team, when you look at the code, is pretty clear that the method returns, well, nothing ;)

Not sure is this is the ruby way, but has being working so far.