export class MutableStack<A> {
   private array: Array<A> = [];
   push(value: A): void {
      this.array.push(value);
   }
   pop(): A | undefined {
      return this.array.pop();
   }
   peek(): A | undefined {
      return this.array.length > 0 ? this.array[this.array.length - 1] : undefined;
   }
   get isEmpty(): boolean {
      return this.array.length === 0;
   }
   get size(): number {
      return this.array.length;
   }
}