concerns chapter

src/SUMMARY.md

@@ -1,3 +1,8 @@
 # Summary
 
 [Introduction](./ch00/s00-introduction.md)
+
+- [Background]()
+- [Implementation]()
+- [Usage]()
+- [Current Implementation Concerns](./ch04/s00-concerns.md)

src/ch04/s00-concerns.md

@@ -0,0 +1,6 @@
+# Concerns (questions) with the current implementaion
+
+## `Mentionable` isn't `Clone`
+* It's meant to be `Rc`d.
+  * For proper indirection, there is `Point`.
+* It can be re-parsed.