input: from typing import List
  
  def str_to_lower(strings: List[str]) -> List[str]:
      """ Convert each string in the list to its lowercase version.
      >>> str_to_lower(['HELLO', 'WoRlD'])
      ['hello', 'world']
      >>> str_to_lower(['THIS', 'is', 'a', 'TeST'])
      ['this', 'is', 'a', 'test']
      """
output:     return [s.lower() for s in strings]