# module A = struct end;;
module A : sig  end

# module A = struct let a = 4 end;;
module A : sig val a : int end
# A.a;;
- : int = 4

# module A = struct let a = 4 let b = a end;;
module A : sig val a : int val b : int end
# A.b;;
- : int = 4

# module A = struct let a = 4 and b = a end;;
module A : sig val a : int val b : int end
# A.b;;
Error: Unbound value a

# module B = A;;
module B : sig val a : int val b : int end
# B.b;;
- : int = 4

# if true then (); 3 else 4;;
Syntax error
# if true then ((); 3) else 4;;
- : int = 3
I think the first expression is correct according to the BNF.

let b = 9 in module A = struct let a = 4 and b = a end;;

open Num;;