Get a value from the keyword-value alist produced by extract-keywords, with default-value support, and additionally return a flag
saying whether the key was bound. Returns (mv value boundp).
Definitions and Theorems
(defun getarg+ (arg default alist)
(declare (xargs :guard (and (eqlablep arg) (alistp alist))))
(b* ((look (assoc arg alist)))
(if look (mv (cdr look) t)
(mv default nil))))