449. A law that has been promulgated may fail to obtain force in the following ways: (a) through contrary custom, already existing and not excluded by the law, or then arising to abrogate the law (see 391 Sqq.); (b) through appeal entered with the lawgiver. Thus, if a bishop deems a law of the Pope unsuited to his diocese, he explains the reasons to the Holy See, and pending the answer it is considered that the lawgiver does not wish the law to oblige. 450. Irritant Laws. Laws Based on Presumption.--There are two classes of human laws that deserve particular mention on account of special difficulties regarding them: (a) irritant laws, which would seem to be unjust, since they declare null what according to natural law would be valid; (b) laws based on presumption, which would seem to be of uncertain force, since presumptions are often contrary to fact. 451. An irritant or inhabilitating law is one that expressly or equivalently declares that certain defects make an act void or voidable, or a person incapable. Such laws are just, even when made by human authority, since it is the common good that makes them necessary, and the natural law itself requires that the common good be promoted. 452. Irritant laws are of various kinds. (a) They are morally or juridically irritant, according as that which is taken from the irritated act is either the natural value it has in conscience, or the positive value it derives from the law. Hence, an act may be legally null (i.e., have no value that the law recognizes or protects) and at the same time morally valid (i.e., of just as much force in conscience as though no irritant law existed). (b) Irritant laws are merely irritant or irritant and prohibitive, according as they make an act invalid but not illicit, or both invalid and illicit. Thus, a law that requires certain formalities for making a will invalidates the act of writing an informal will, but does not make it an offense; but the church law of diriment impediments makes a marriage contracted with one of these impediments both null and sinful. (c) Irritant laws are merely irritant or irritant and penal, according as the legislator does not or does intend them as punishments. For example, the law of clandestinity is merely irritant; the law regarding the impediment of crime is probably both irritant and penal. 453. Laws that are merely irritant do not oblige one in conscience to omit the act, but only to suffer the effect of irritation; but laws that are both irritant and prohibitive oblige one in conscience to omit the act. Example: In itself, it is not unlawful to make an informal will, but it is unlawful to marry with a diriment impediment. 454. As to the time when irritant laws obtain their effect, the following points are important.