adding assert for better error

lang/README.md

@@ -633,6 +633,30 @@ main := fn(): uint {
 
 ### Purely Testing Examples
 
+#### triggering_store_in_divergent_branch
+```hb
+opaque := fn(): uint {
+	return 1 << 31
+}
+
+main := fn(): void {
+	a := 0
+	loop if a >= opaque() break else {
+		valid := true
+		b := 0
+		loop if b >= opaque() break else {
+			if b == 1 << 16 {
+				valid = false
+				break
+			}
+			b += 1
+		}
+		if valid == false continue
+		a += 1
+	}
+}
+```
+
 #### very_nested_loops
 ```hb
 $W := 200

lang/src/son.rs

@@ -4823,6 +4823,7 @@ mod tests {
         fb_driver;
 
         // Purely Testing Examples;
+        triggering_store_in_divergent_branch;
         wrong_dead_code_elimination;
         memory_swap;
         very_nested_loops;

lang/src/utils.rs

@@ -130,6 +130,7 @@ impl BitSet {
         }
 
         let (elem, bit) = Self::indexes(index);
+        debug_assert!(elem < data.len(), "{} < {}", elem, data.len());
         let elem = unsafe { data.get_unchecked_mut(elem) };
         let prev = *elem;
         *elem |= 1 << bit;