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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def first_key(lines):
 # type: (List[str]) -> str """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. # some comment 'foo': true 'foo' """

    for line in lines:
        if line.startswith('#'):
            continue
        if any(line.startswith(quote) for quote in QUOTES):
            return line[1:]
        return line
    else:
        return ''