move stuff around

library/internal/container.ts

@@ -0,0 +1,27 @@
+export class Container<T> {
+  private items: T[] = []
+
+  add(...items: T[]) {
+    this.items.push(...items)
+  }
+
+  addBefore(item: T, before: T) {
+    let index = this.items.indexOf(before)
+    if (index === -1) {
+      index = this.items.length
+    }
+    this.items.splice(index, 0, item)
+  }
+
+  remove(toRemove: T) {
+    this.items = this.items.filter((item) => item !== toRemove)
+  }
+
+  clear() {
+    this.items = []
+  }
+
+  [Symbol.iterator]() {
+    return this.items[Symbol.iterator]()
+  }
+}