;;--------------------=={ Remove Once }==---------------------;;
;;                                                            ;;
;;  Removes the first occurrence of an item from a list       ;;
;;------------------------------------------------------------;;
;;  Author: Lee Mac, Copyright © 2011 - www.lee-mac.com       ;;
;;------------------------------------------------------------;;
;;  Arguments:                                                ;;
;;  x - item to be removed                                    ;;
;;  l - list from which to remove first occurrence of item    ;;
;;------------------------------------------------------------;;
;;  Returns:  Resultant list following the removal            ;;
;;------------------------------------------------------------;;

(defun LM:RemoveOnce ( x l )
    (if l
        (if (equal x (car l))
            (cdr l)
            (cons (car l) (LM:RemoveOnce x (cdr l)))
        )
    )
)

;;--------------------=={ Remove Once }==---------------------;;
;;                                                            ;;
;;  Removes the first occurrence of an item from a list       ;;
;;------------------------------------------------------------;;
;;  Author: Lee Mac, Copyright © 2011 - www.lee-mac.com       ;;
;;------------------------------------------------------------;;
;;  Arguments:                                                ;;
;;  x - item to be removed                                    ;;
;;  l - list from which to remove first occurrence of item    ;;
;;------------------------------------------------------------;;
;;  Returns:  Resultant list following the removal            ;;
;;------------------------------------------------------------;;

(defun LM:RemoveOnce ( x l / f )
    (setq f equal)
    (vl-remove-if '(lambda ( a ) (if (f a x) (setq f (lambda ( a b ) nil)))) l)
)

_$ (LM:RemoveOnce 3 '(1 2 3 4 5 1 2 3 4 5))
(1 2 4 5 1 2 3 4 5)