I would like to suggest to make Dibi\Expression class into an interface, as there is currently no option to use anything else than inherit Expression and am afraid it will become final in the future without means to extend/replace... Also, the use of private $values is a hindrance.

Currently, it's only used inside the Translator class with instanceof operator, which si an ideal situation to turn it into an interface.

IExpression or Expressable.

I'll gladly provide a PR.