forgot to add generated id to used ids

src/main/java/betterquesting/api2/storage/SimpleDatabase.java

@@ -5,17 +5,24 @@
 public class SimpleDatabase<T> extends AbstractDatabase<T> {
 
     private final IntOpenHashSet ids = new IntOpenHashSet();
+    /**
+     * the "smallest acceptable" id, any id smaller than this is considered as used and can
+     * be skipped when searching for new id, this condition can be maintained by:
+     * - refresh lowerBound after new id is generated: {@link #nextID()}
+     * - refresh lowerBound after one id is removed: {@link #removeID(int)}
+     */
     private int lowerBound = 0;
 
     @Override
     public synchronized int nextID() {
         for (int i = lowerBound; i < Integer.MAX_VALUE ; i++) {
             if (!ids.contains(i)) {
+                ids.add(i);
                 lowerBound = i + 1;
                 return i;
             }
         }
-        throw new IllegalStateException(String.format("All integer id from 0 to %s have been consumed", Integer.MAX_VALUE));
+        throw new IllegalStateException(String.format("All integer id from 0 to %s have been consumed, it's abnormal", Integer.MAX_VALUE));
     }
 
     @Override