input: from typing import Callable
  
  def to_lower(string: str) -> str:
      """ Converts all characters of a given string to lowercase.
      >>> to_lower("HELLO")
      'hello'
      >>> to_lower("HeLLo")
      'hello'
      >>> to_lower("hello")
      'hello'
      """
      return string.lower()
output:     return string.lower()