input: from typing import List
  
  
  def to_lower(string: List[str]) -> str:
      """ Convert all characters of a given string to lowercase.
  
      >>> to_lower("HELLO")
      'hello'
      """
      return string.lower()
output:     result = []
      for s in string:
          result.append(s.lower())
      return ''.join(result)