[petercooper]

I find it's best to just blindly accept it in Ruby, because you can really go down the rabbit hole otherwise. For example, the first example has even more syntactic fun going on because it looks like `map` accepts an argument but it doesn't:

    irb(main)> [1,2,3].map(1)
    (irb): in `map': wrong number of arguments (given 1, expected 0)

So you get clever and think.. well, it accepts a Proc, clearly, in the shape of &:operation. Let's use a Proc:

    irb(main)> [1,2,3].map(Proc.new { })
    (irb):in `map': wrong number of arguments (given 1, expected 0)

In the context of blocks, & is doing a little more than merely providing a Proc, but it's a handy way to think about it.