Code snippet in chapter "Variables and Functions" exercises scoping and shadowing

[reynir]

Someone on IRC asked a question about the following snippet of code:

examples/code/variables-and-functions/main.topscript

Lines 183 to 186 in 32ea926

	let concat ?sep x y =
	let sep = match sep with None -> "" | Some x -> x in
	x ^ sep ^ y
	;;

I noticed the snippet exercises non-recursive let-bindings, scoping, and shadowing, and I think a newcomer would easily find it confusing to read. The user mentioned it might have been to reinforce earlier-learned concepts. If so, then maybe it would be a good idea to mention this :-)

Relevant logs from IRC: https://irclog.whitequark.org/ocaml/2018-05-23#1527056651-1527058456;