pattern-matching: add explanatory commentary to solution

src/pattern-matching/solution.md

@@ -3,3 +3,25 @@
 ```rust,editable
 {{#include exercise.rs:solution}}
 ```
+
+- **Pattern Matching:** We use `match` to handle the different variants of the
+  `Expression` enum. This ensures we cover all possible cases.
+- **Destructuring:** The pattern `Expression::Op { op, left, right }`
+  destructures the `Op` variant, giving us access to its inner fields.
+- **Recursion:** Since `Expression` is a recursive data structure, `eval` is a
+  recursive function. We call `eval(*left)` and `eval(*right)` to compute the
+  values of the sub-expressions.
+- **Smart Pointers (`Box`):** The `left` and `right` fields are of type
+  `Box<Expression>`. `Box` puts the value on the heap. The `*` operator
+  dereferences the box, moving the inner `Expression` out so it can be passed by
+  value to the recursive `eval` calls.
+
+<details>
+
+- Mention that `Box` is necessary because `Expression` has infinite size
+  otherwise (it contains itself). `Box` provides a fixed size (pointer size) for
+  the recursive fields.
+- Discuss integer division behavior (truncation) for `Operation::Div` since we
+  are using integers.
+
+</details>