Find unused facts

[inevolin]

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. :)