input: def classify_emails(X_train, y_train, X_test):
      """ Construct a naive bayes classifier using python to classify emails as spam or not spam.
      >>> model = classify_emails(X_train, y_train, X_test)
      >>> y_pred = model.predict(X_test)
      """
      from sklearn.naive_bayes import GaussianNB
      
      # Create a Gaussian Classifier
      model = GaussianNB()
      
      # Train the model using the training sets 
      model.fit(X_train, y_train)
      
      return model
output:     return model