@system pe=2, sm=1

$fib |> {
  ; ── function body ──
  &n <| pass
  lt &n, 2 |> &test
  &test <| sweq
  &n |> &test:L

  ; base case
  &test:L |> @ret

  ; recursive case
  sub &n, 1 |> &n1
  sub &n, 2 |> &n2

  ; two calls, each 3 monadic IRAM slots + shared stub
  &__alloc1  <| rd_inc, @ctx_alloc
  &__exec1   <| exec, @fib_call_seq
  &__extag1  <| extract_tag, 20          ; results arrive at offset 20

  &__alloc1  |> &__fib_ctx_fan
  &__extag1  |> &__fib_ct_ret:R
  &n1        |> &__fib_ct_n:R

  &__alloc2  <| rd_inc, @ctx_alloc
  &__exec2   <| exec, @fib_call_seq
  &__extag2  <| extract_tag, 21          ; results arrive at offset 21

  &__alloc2  |> &__fib_ctx_fan
  &__extag2  |> &__fib_ct_ret:R
  &n2        |> &__fib_ct_n:R

  ; reduction
  add &r1, &r2 |> @ret                   ; r1 at offset 20, r2 at offset 21
}