Add multi-valued-logic.md

rust/multi-valued-logic.md

+In Rust, it's pretty easy to create a logical type that can encode "unknown" or 
+"indeterminate" system.
+
+```
+type TernaryLogic = Option(bool);
+```
+
+This approach is also available in Go and other languages with pointers:
+
+```
+*bool
+```
+
+When de-referencing the pointer, `nil` equals unknown and `true`/`false` are represented.
+
+I'm not actually sure how the compiler can distinguish between a null pointer
+and false. Something more to learn, I guess.
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