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class (Eq a) => Graph a where
    vert :: [a]
    ady  :: a -> [a]
    
class (Graph a) => Paths a where
    gps    :: a -> a -> [[a]]
    
class (Paths a) => Minimum a where
    mgps   :: a -> a -> [a]
    
instance Minimum Char where
    vert = ['a' .. 'f'] -- This is the line of the error message.
    ady 'a' = ['b', 'c']
    ady 'b' = ['d', 'c', 'e']
    ady 'd' = ['f']
    gps = bepgr -- bepgr is defined and implemented in other code's part
    mgps = head gps
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