>>> cons a [] [a]
>>> cons a [b, c] [a,b,c]
>>> cons a (Seq.fromList []) fromList [a]
>>> cons a (Seq.fromList [b, c]) fromList [a,b,c]
cons 't' "est" == "test" \x xs -> uncons (cons x xs) == Just (x,xs)
toNonEmpty (cons x xs) = cons x (toNonEmpty xs)
> 4 `cons` [1,2,3] [4,1,2,3]