input: Write a function to create a Flask web application that takes user input and generates an HTML page displaying the input. The function should return the Flask app instance.
  assert create_flask_app().__name__=='__main__'
output: from flask import Flask, request, render_template_string
  
  def create_flask_app():
      app = Flask(__name__)
  
      @app.route('/', methods=['GET', 'POST'])
      def index():
          if request.method == 'POST':
              user_input = request.form['user_input']
              html_content = f"<h1>You entered: {user_input}</h1>"
              return render_template_string(html_content)
          return '''
              <form method="post">
                  <input type="text" name="user_input">
                  <input type="submit">
              </form>
          '''
  
      return app