ghci> [[[1,6,[1,9,0,9],6]]] :: P
L [L [L [N 1,N 6,L [N 1,N 9,N 0,N 9],N 6]]]
ghci> (fromList . read $ "[[[1,6,[1,9,0,9],6]]]") :: P
L *** Exception: Prelude.read: no parse