Just FYI, I found a source saying that R5RS defines (though I could not find it
there) 'letrec' via this transformation (which also answers my question):
(letrec ([x1 e1] ... [xn en]) body)
~>
(let ([x1 undefined] ... [xn undefined])
(let ([t1 e1] ... [tn en])
(set! x1 t1)
...
(set! xn tn))
body)Hans
