. . .
> This is not universally true:
> 
> > # let y = 1
> >   and z = y + 2 in
> >   z;;
> > - : int = 3
> 
>             Objective Caml version 3.08.1
> 
>     # let y = 1
>       and z = y + 2 in
>       z;;
>     Unbound value y
> 
> So either you are using a version older than 3.08.1 or this is a fairly
> recent change.  In the latter case, people who wish to remain backward-
> compatible might eschew this style for sequential bindings, regardless
> of any potential performance problems.

I'm using Objective Caml version 3.08.3 and had this interaction:
    # let y = 1
      and z = y + 2 in
        z;;
    Unbound value y
    #

 -- Bill Wood
    bill.wood@acm.org