input: Write a python function to create a user and make a post on a social media platform.
  assert create_user_and_post("john_doe", "Hello World!") == "Hello World!"
  assert create_user_and_post("jane_doe", "This is a test post.") == "This is a test post."
  assert create_user_and_post("john_doe", "Another post by John.") == "Another post by John."
output: def create_user_and_post(username, post_content):
      users = {}
  
      def create_user(username):
          if username not in users:
              users[username] = []
          return username
  
      def make_post(username, post_content):
          users[username].append(post_content)
          return post_content
  
      create_user(username)
      return make_post(username, post_content)