Add reverse case function

manuscript/chapter1.txt

@@ -209,6 +209,32 @@ $ upper "HELLO"
 HELLO
 ```
 
+## Reverse a string case
+
+**CAVEAT:** Requires `bash` 4+
+
+**Example Function:**
+
+```sh
+reverse_case() {
+    # Usage: reverse_case "string"
+    printf '%s\n' "${1~~}"
+}
+```
+
+**Example Usage:**
+
+```shell
+$ reverse_case "hello"
+HELLO
+
+$ reverse_case "HeLlO"
+hElLo
+
+$ reverse_case "HELLO"
+hello
+```
+
 ## Trim quotes from a string
 
 **Example Function:**