Why not returning `false` here ? In itself, the `asserter` class does not handle anything.
I would rather return `false` here and override this method somewhere in `asserters/variable` with a truthy retur value.

This is only OK if every native type asserter inherit from `asserter/variable`... I don't rememeber exactly if it's the case or not. Also, this is not a big deal as it will only avoid a bit of duplication on the code.