let rec sum n =
  if n = 0 then 0 else n + sum (n - 1)

let goodSum n =
  let rec helper acc n =
    if n = 0 then acc else helper (n + acc) (n - 1) in
  helper 0 n



helper 0 4 ==> helper (4 + 0) (4 - 1)
           ==> helper 4 3
           ==> helper (3 + 4) 2
           ==> helper 7 2
           ==> helper (2 + 7) 1
           ==> helper 9 1
           ==> helper (1 + 9) 0
           ==> helper 10 0
           ==> 10

let doTwice (f : 'b) (x : 'a) = f (f x)
1. f must be a function
   So: 'b = 'c -> 'd
2. f is taking x as input
   So: 'c = 'a

doTwice (f : 'a -> 'd) (x : 'a) = f (f x)
                                    -----

3. f is itself taking f x as input
                      --- : 'd
4. So, 'a = 'd

doTwice (f : 'a -> 'a) (x : 'a) : 'a = f (f x)
doTwice : ('a -> 'a) -> 'a -> 'a

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HW1 due on Friday next week, 10pm.
Quiz 1 moved by one week: Tuesday, October 8.