Variable binding

# let x = 3;;
val x : int = 3

# let y = 5;;
val y : int = 5

# let z = x+y;;
val z : int = 8

# let x = 1000;;
val x : int = 1000

# z;;
- : int = 8

# let a = 3 in
  let b = 5 in
  a+b;;
- : int = 8

# a;;
Characters 0-1:
  a;;
  ^
Unbound value a