code: def first_key(lines):  # type: (List[str]) -> str
      
      for line in lines:
          if line.startswith('#'):
              continue
          if any(line.startswith(quote) for quote in QUOTES):
              return line[1:]
          return line
      else:
          return ''
docs: Returns a string representing the sort key of a block.
  
      The sort key is the first YAML key we encounter, ignoring comments, and
      stripping leading quotes.
  
      >>> print(test)
      # some comment
      'foo': true
      >>> first_key(test)
      'foo'