Comment by jameshart

Also: forgot the evolution into its final form, which is a list of rule objects.

A practical example:

Your program starts out accepting a config file that looks like this:

    enableLogging: true

Then it changes into:

    logLevel: WARN

Then that becomes:

    logging: {
        level: WARN
        file: system.log
    }

Then:

    logging: 
        fileLogger: {
            level: WARN
            file: system.log
        }

And finally:

    logging: 
        - fileLogger: {
            level: WARN
            file: system.log
        }
        - consoleLogger: {
            level: DEBUG
        }