input: 
  def classify_string(input_string: str) -> str:
      """
      Given an input string, this function uses a machine learning classifier to 
      categorize it. The function assumes that feature extraction and transformation
      from string to numeric are already defined and implemented.
  
      >>> classify_string("Hello, how are you today?")
      'greeting'
      """
  
output:     features = extract_features(input_string)
      transformed_features = transform_features(features)
      prediction = classifier.predict([transformed_features])[0]
      return prediction