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Formal-nth

The definition of (NTH n lst) for integers n and formal terms lst.

Definitions and Theorems

Function: formal-nth

(defun formal-nth (n lst)
  (declare (xargs :guard (and (integerp n)
                              (<= 0 n)
                              (pseudo-termp lst)
                              (equal (formal-true-listp lst) *t*))))
  (case-match lst
    (('quote x)
     (cons 'quote (cons (nth n x) 'nil)))
    (& (cond ((zp n) (fargn lst 1))
             (t (formal-nth (- n 1) (fargn lst 2)))))))