diff --git a/auxilary.ts b/auxilary.ts
new file mode 100644
index 0000000..1c556b1
--- /dev/null
+++ b/auxilary.ts
@@ -0,0 +1,38 @@
+export class TwoWayMap<A, B> {
+  private mapAtoB = new Map<A, Set<B>>();
+  private mapBtoA = new Map<B, Set<A>>();
+
+  // prepare
+  private prepareA(a: A) {
+    if (this.mapAtoB.has(a)) return;
+    this.mapAtoB.set(a, new Set());
+  }
+
+  private prepareB(b: B) {
+    if (this.mapBtoA.has(b)) return;
+    this.mapBtoA.set(b, new Set());
+  }
+
+  // utility
+  private getListOnA(a: A) {
+    return this.mapAtoB.get(a);
+  }
+
+  private getListOnB(b: B) {
+    return this.mapBtoA.get(b);
+  }
+
+  // public
+  set(a: A, b: B) {
+    this.prepareA(a);
+    this.prepareB(b);
+
+    this.getListOnA(a)?.add(b);
+    this.getListOnB(b)?.add(a);
+  }
+
+  delete(a: A, b: B) {
+    this.getListOnA(a)?.delete(b);
+    this.getListOnB(b)?.delete(a);
+  }
+}