Variable processors

Variable files are processed first. Variable processors are written in the same format, but unlike variable files, they do not overwrite existing variables.

Let's say you're using Prove to set up an application that has library dependencies. The version of said dependencies will vary depending on the verison of the application. You want the dependency verisons to be set depending on the application version by default, but you also want to be able to override the dependency version on a per-installation basis when necessary.

To solve this, you place the application version in the variables, and the default dependency versions are defined in variable processors.

Variable processors could be tied to states, meaning a variable processor is automatically invoked before the state file(s) are invoked.