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. :)

該 issue 未指明檔案或測試；從進入點 engine(rule).run(facts) 開始，追蹤 facts 如何根據 rule 進行評估。定義應如何回報未使用的 facts（例如 B、C 和 D），然後為 pseudocode 情境新增覆蓋。

由索引模型根據 Issue 內容生成。