diff --git a/bag.ts b/bag.ts
new file mode 100644
index 0000000..53ed2dc
--- /dev/null
+++ b/bag.ts
@@ -0,0 +1,77 @@
+// Bags and Deltas (which are Bags where item-counts can be negative).
+
+import { Value, Set, Dictionary, DefaultPointer } from 'preserves';
+
+export enum ChangeDescription {
+    PRESENT_TO_ABSENT = -1,
+    ABSENT_TO_ABSENT = 0,
+    ABSENT_TO_PRESENT = 1,
+    PRESENT_TO_PRESENT = 2,
+}
+
+export class Bag<T extends object = DefaultPointer> {
+    _items: Dictionary<number, T>;
+
+    constructor(s?: Set<T>) {
+        this._items = new Dictionary();
+        if (s) s.forEach((v) => this._items.set(v, 1));
+    }
+
+    get(key: Value<T>): number {
+        return this._items.get(key, 0) as number;
+    }
+
+    change(key: Value<T>, delta: number, clamp: boolean = false): ChangeDescription {
+        let oldCount = this.get(key);
+        let newCount = oldCount + delta;
+        if (clamp) {
+            newCount = Math.max(0, newCount);
+        }
+
+        if (newCount === 0) {
+            this._items.delete(key);
+            return (oldCount === 0)
+                ? ChangeDescription.ABSENT_TO_ABSENT
+                : ChangeDescription.PRESENT_TO_ABSENT;
+        } else {
+            this._items.set(key, newCount);
+            return (oldCount === 0)
+                ? ChangeDescription.ABSENT_TO_PRESENT
+                : ChangeDescription.PRESENT_TO_PRESENT;
+        }
+    }
+
+    clear() {
+        this._items = new Dictionary();
+    }
+
+    includes(key: Value<T>): boolean {
+        return this._items.has(key);
+    }
+
+    get size(): number {
+        return this._items.size;
+    }
+
+    keys(): IterableIterator<Value<T>> {
+        return this._items.keys();
+    }
+
+    entries(): IterableIterator<[Value<T>, number]> {
+        return this._items.entries();
+    }
+
+    forEach(f: (count: number, value: Value<T>) => void) {
+        this._items.forEach(f);
+    }
+
+    snapshot(): Dictionary<number, T> {
+        return this._items.clone();
+    }
+
+    clone(): Bag<T> {
+        const b = new Bag<T>();
+        b._items = this._items.clone();
+        return b;
+    }
+}