>From: Claudio Sacerdoti Coen <sacerdot@students.cs.unibo.it>
[...]
>can you explain me why is this not allowed?
>I can't see a problematic counterexample.
[...]
># let rec id'' = id;;
>This kind of expression is not allowed as right-hand side of `let rec'

Because we don't know how to compile "let rec x = x" or
  let rec x = y
  and y = x

Moreover, you can just remove the "rec" and it works.  It is possible
to implement "let rec var1 = var2", but the usefulness is quite small
compared to the amount of code we would need to write.

-- Damien