ranks = [('black', 'dark'), ('black', 'dim'), ('black', 'gloomy'), ('dark',
    'gloomy'), ('dim', 'dark'), ('dim', 'gloomy')]
max(paths(ranks), key=len)
ranks = [('a', 'c'), ('b', 'a'), ('b', 'c'), ('d', 'a'), ('d', 'b'), ('d', 'c')
    ]
max(paths(ranks), key=len)
Given a linear order completely represented by a list of tuples of strings, output the order as a list of strings