Do-loop enigma with two variables

[Pierre Lairez]

Dear guile users,

When running the following loop:
(do ((i 1 (+ 1 i))
     (j 0 i))
    ((> i 4) (newline))
  (display (list i j)))

I expect without hesitation to read
(1 0)(2 1)(3 2)(4 3)

To my surprise, I obtain
(1 0)(2 2)(3 3)(4 4)

After macro-expansion, the loop above is rewritten in the following:
(letrec ((loop
          (λ (i j)
            (if (> i 4)
                (newline)
                (begin
                  (display (list i j))
                  (loop (+ 1 i) i))))))
  (loop 1 0))

The equality j = i + 1 is clearly a loop invariant, the function “loop“
is *never* called with two equal arguments. So I cannot understand why
(2 2) may possibly appear in the output. What do I not see?

Best regards,

Pierre

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