Email Subscription

Implement a system to manage users' email subscriptions. Requirements: Implement functionality to add users to a subscription list. Implement functionality to remove users from the subscription list. Implement functionality to send emails to all users in the subscription list. Input Data: Add User: ADD <email> Remove User: REMOVE <email> Send Email: SEND <message> Output Data: On success adding or removing a user, output SUCCESS. When attempting to remove a non-existing user, output USER NOT FOUND. After all users have received the email, output EMAIL SENT TO <number_of_recipients> USERS. Constraints: User email is a unique identifier. Provided emails are valid addresses. Example Input/Output: Input: ADD user1@example.com ADD user2@example.com SEND Hello, World! REMOVE user1@example.com SEND Goodbye! Output: SUCCESS SUCCESS EMAIL SENT TO 2 USERS SUCCESS EMAIL SENT TO 1 USERS Example Input ADD user1@example.com ADD user2@example.com SEND Hello, World! REMOVE user1@example.com SEND Goodbye!