Gnus-remove-from-range

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(defun gnus-remove-from-range (range1 range2)
  "Return a range that has all articles from RANGE2 removed from
RANGE1. The returned range is always a list. RANGE2 can also be a
unsorted list of articles. RANGE1 is modified by side effects, RANGE2
is not modified."
  (if (or (null range1) (null range2))
      range1
    (let (out r1 r2 r1_min r1_max r2_min r2_max
	      (range2 (gnus-copy-sequence range2)))
      (setq range1 (if (listp (cdr range1)) range1 (list range1))
	    range2 (sort (if (listp (cdr range2)) range2 (list range2))
			 (lambda (e1 e2)
			   (< (if (consp e1) (car e1) e1)
			      (if (consp e2) (car e2) e2))))
	    r1 (car range1)
	    r2 (car range2)
	    r1_min (if (consp r1) (car r1) r1)
	    r1_max (if (consp r1) (cdr r1) r1)
	    r2_min (if (consp r2) (car r2) r2)
	    r2_max (if (consp r2) (cdr r2) r2))
      (while (and range1 range2)
	(cond ((< r2_max r1_min)                           ; r2 < r1
	       (pop range2)
	       (setq r2 (car range2)
		     r2_min (if (consp r2) (car r2) r2)
		     r2_max (if (consp r2) (cdr r2) r2)))
	      ((and (<= r2_min r1_min) (<= r1_max r2_max)) ; r2 overlap r1
	       (pop range1)
	       (setq r1 (car range1)
		     r1_min (if (consp r1) (car r1) r1)
		     r1_max (if (consp r1) (cdr r1) r1)))
	      ((and (<= r2_min r1_min) (<= r2_max r1_max)) ; r2 overlap min r1
	       (pop range2)
	       (setq r1_min (1+ r2_max)
		     r2 (car range2)
		     r2_min (if (consp r2) (car r2) r2)
		     r2_max (if (consp r2) (cdr r2) r2)))
	      ((and (<= r1_min r2_min) (<= r2_max r1_max)) ; r2 contained in r1
	       (if (eq r1_min (1- r2_min))
		   (push r1_min out)
		 (push (cons r1_min (1- r2_min)) out))
	       (pop range2)
	       (if (< r2_max r1_max) ; finished with r1?
		   (setq r1_min (1+ r2_max))
		 (pop range1)
		 (setq r1 (car range1)
		       r1_min (if (consp r1) (car r1) r1)
		       r1_max (if (consp r1) (cdr r1) r1)))
	       (setq r2 (car range2)
		     r2_min (if (consp r2) (car r2) r2)
		     r2_max (if (consp r2) (cdr r2) r2)))
	      ((and (<= r2_min r1_max) (<= r1_max r2_max)) ; r2 overlap max r1
	       (if (eq r1_min (1- r2_min))
		   (push r1_min out)
		 (push (cons r1_min (1- r2_min)) out))
	       (pop range1)
	       (setq r1 (car range1)
		     r1_min (if (consp r1) (car r1) r1)
		     r1_max (if (consp r1) (cdr r1) r1)))
	      ((< r1_max r2_min)                           ; r2 > r1
	       (pop range1)
	       (if (eq r1_min r1_max)
		   (push r1_min out)
		 (push (cons r1_min r1_max) out))
	       (setq r1 (car range1)
		     r1_min (if (consp r1) (car r1) r1)
		     r1_max (if (consp r1) (cdr r1) r1)))))
      (when r1
	(if (eq r1_min r1_max)
	    (push r1_min out)
	  (push (cons r1_min r1_max) out))
	(pop range1))
      (while range1
	(push (pop range1) out))
      (nreverse out))))