I don't think this name implies we're converting the argument into our currency. What if we added an argument to `#fractional` like below that also converts it? I'm going to use ruby2.0 keyword args in the example. We'd need to use

```ruby
def fractional(exchange_to: self.currency)
  fraction = self.exchange_to(exchange_to).instance_variable_get(:@fractional)

  # ... same as before ...
end
```

Thoughts?