SBCL tells me it is deleting unreachable code that is almost surely reachable

Here is a revision of a function in hayat. The revision eliminates or helps to eliminate two testsuite failures.

The SBCL (2.4.7)  compiler tells me that

; file: C:/Users/barton/maxima-code-pure/maxima-code/src/./hayat.lisp
; in: DEFUN COEF-SIGN
;     (EQ MAXIMA::SGN 'MAXIMA::$POS)
;
; note: deleting unreachable code

I think that is rubbish—likely, I'll commit the (commented out) memq version.

(defun coef-sign (coef)

  (let ((sgn ($csign coef)))
    (cond ((or (eq sgn '$complex) (eq sgn '$imaginary)) '$im)
          ;((memq sgn '($neg '$zero '$pos)) sgn)
          ((or (eq sgn '$neg) (eq sgn '$zero) (eq sgn '$pos)) sgn)
          (t
             (let ((asgn ($asksign coef)))
                ;; When sgn is zero and limit is active, record this fact as a noun equality
                ;; in *limit-assumptions*.
             (when (and *limit-assumptions* (eq asgn '$zero))
               (push (ftake '$equal coef 0) *limit-assumptions*))
            asgn)))))

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