let x = 12;;
let y = 13;;
let x = 25;;
x;;
Nil;;
[];;
1::[];;
2::1::[];;
[[1;2];[4;5]];;
succ;;
pred;;
[succ;pred];;
[1;'1'];;
[1;2;3;4];;
let hd xs = match xs with
 | [] -> failwith "hd: empty list"
 | x::xs -> x;;
hd [1;2;3;4];;
hd [1];;
hd [];;
let hd xs = match xs with
 | x::xs -> x;;
hd [];;
let hd = function
 | x::xs -> x;;
let hd = function
 | x::_ -> x;;
let hd (x::_) = x;;
let hd [];;
hd [];;
hd [1;2;3];;
let fst_or_snd xs = match xs
 | [x] -> x
 | _::y::_ -> y
 | _ -> failwith "empty list";;
let rec replicate n x = 
 if n < 1 then [] else x::replicate (n-1) x;;
replicate 2 'c';;
#trace replicate;;
replicate 2 'c';;
let rec range m n = 
 if m >= n then []
 else m::range (m+1) n;;
(>=);;
(+);;
range 1 3;;
range 1 100;;
range 1 1000;;
range 1 100000;;
range 1 1000000;;
let rec range m n = 
 if m >= n then []
 else m::range m n;;
range 1 3;;
let rec sum = function 
 | [] -> 0
 | x::xs -> x + sum xs;;
let rec prod = function 
 | [] -> 1
 | x::xs -> x * prod xs;;
prod [1;2;3;4;5];;
let rec foldr f b xs = match xs with
 | [] -> b
 | x::xs -> f x (foldr f b xs);;
let sum xs = foldr (+) 0 xs;;
sum [1;2;3;4;5];;
let prod xs = foldr ( * ) 1 xs;;
prod [1;2;3;4;5];;
let (@) xs ys = match xs with  [] -> ys | x::xs -> x::(xs@ys);;
let rec foldr = function [] -> b | x::xs -> f x (foldr f b xs);;
let rec foldr f b = function [] -> b | x::xs -> f x (foldr f b xs);;