In case you missed it, this is not a programming language parser/validator: it's to validate data objects in an information system. Validating a form submission would be a good example. It looks to be along similar lines to JSON schema.

From the LIVR docs:

"Rules should be able to change results output"

My opinion is that if you're going to use a declarative validation engine like this then it should not be able to modify the data entity being validated.