Follow the updated SRFI-42

Comment from SRFI;
; Bug #2:
;    Unfortunately, the above expansion is still incorrect (as Jens-Axel
;    Soegaard pointed out, 4-Jun-2007) because ib-rhs are evaluated even
;    if ne1?-value is #f, indicating that the loop has ended.
;       The problem manifests itself in the following example:
;
;      (do-ec (:while (:list x '(1)) #t) (display x))
;
;    Which iterates :list beyond exhausting the list '(1).
;
;    For the fix, we follow Jens-Axel's approach of guarding the evaluation
;    of ib-rhs with a check on ne1?-value.
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