Find unused facts

Is it possible to retrieve a list of facts which were not used / triggered by any rule in the engine?

Pseudo code:

```
facts = {
A: true,
B: false,
C: true,
D: false
}

rule = {
check if A == true ?
}

engine(rule).run(facts)
```

I want to retrieve facts which were not used by this rule (`B, C, D`)

This is an atypical request I guess. :)

The issue names no files or tests; start at the engine(rule).run(facts) entry point and trace how facts are evaluated against a rule. Define how unused facts such as B, C, and D would be reported, then add coverage for the pseudocode scenario.

Written by the indexing model from the issue text.