add case statement type narrowing

content/case.md

@@ -48,4 +48,19 @@ $ nimrod c -r ./case_stmts.nim
 C
 Consonant
 negative
+```
+
+Another feature of sets is that when possible, the possible types are narrowed to a range so that the following code works without requiring an else statement.
+
+``` nimrod
+let x = 2
+case (x and 3) + 7
+of 7: echo "A"
+of 8: echo "B"
+of 9: echo "C"
+of 10: echo "D"
+```
+``` console
+$ nimrod c -r ./case_stmts_1.nim
+C
 ```