prefix xs (y:ys) | isPrefixOf xs (y:ys) = []
                 | otherwise            = y : prefix xs ys
prefix xs (y:ys) | isPrefixOf xs (y:ys) = []
                 | otherwise            = y : prefix xs ys
prefix xs (y:ys) | isPrefixOf xs (y:ys) = []
                 | otherwise            = y : prefix xs ys