:param list[] lines:
    :rtype: str|None

def get_same_indent_prefix(lines):
    """
    :param list[] lines:
    :rtype: str|None
    """
    if not lines:
        return ""
    prefix = get_indent_prefix(lines[0])
    if not prefix:
        return ""
    if all([l.startswith(prefix) for l in lines]):
        return prefix
    return None