down: [16][4]
down = [ 1+d | d <- tail down ] # [0]

down': [16][4]
down' = [ 1+(down' @ i) | i <- [1 .. 15] ] # [0]

take_some: {n,k,t} (n>=k) => [n]t -> [k]t
take_some xs = takes where takes = [ x | x <- xs | _ <- takes ]