-108§ 833. Conditions precedent A condition precedent is one which is to be performed before some right dependent thereon accrues, or some act dependent thereon is performed. § 834. Conditions concurrent Conditions concurrent are those which are mutually dependent, and are to be performed at the same time. § 835. Conditions subsequent A condition subsequent is one referring to a future event, upon the liappening of which the obligation becomes no longer binding upon f he other party, if he chooses to avail himself of the condition. § 836. Performance of conditions necessary Before a party to an obligation can require another party to perform any act under it, he shall fulfill all conditions precedent thereto imposed upon himself and be able and offer to fulfill all conditions concurrent so imposed upon him on the like fulfillment by the other party, except as provided by section 837 of this title. § 837. Performance excused If a party to an obligation gives notice to another, before the latter is in default, that he will not perform the same upon his part, and does not retract the notice before the time when performance upon his part is due, the other party is entitled to enforce the obligation without previously performing or offering to perform any conditions upon his part in favor of the former party. § 838. Impossible or unlawful conditions A condition in a contract, the fulfillment of which is impossible or unlawful within the meaning of sections 1081-1085 of this title, or which is repugnant to the nature of the interest created by the contract, is void. § 839. Construction of conditions involving forfeiture A condition involving a forfeiture shall be strictly interpreted against the party for whose benefit it is created. Article D—Alternative Obligations § 851. Right of selection If an obligation requires the performance of one of two acts, in the alternative, the party required to perform has the right of selection, unless it is otherwise provided by the terms of the obligation. § 852. Loss of right of selection If the party having the right of selection between alternative acts does not give notice of his selection to the other party within the time, if any, fixed by the obligation for that purpose, or, if none is so fixed, before the time when the obligation ought to be performed, the right of selection passes to the other party. § 853. Indivisibility of alternatives A party having the right of selection between alternative acts shall select one of them in its entirety, and may not select part of one and part of another without the consent of the other party. § 854. Nullity of alternatives If one of the alternative acts required by an obligation is such as the law will not enforce, or becomes unlawfm, or impossible of performance, the obligation is to be interpreted as though the other stood alone.