In 681 BC, the Assyrian King Sennacherib was assassinated by one of his sons. In Babylon, his death was seen as divine punishment for his destruction of that city in 689 BC.

A different type of patricide occurred in EPO Board of Appeal Decision T1496/11 when the Board concluded that claim 1 of a parent application lacked novelty over an embodiment described in its own divisional application, which was entitled to an earlier priority date. Hence, claim 1 of the parent was destroyed by its own divisional child.

How could this happen? In the decision T1496/11, the invention concerned banknotes comprising a transparent window that could function as a lens so that, when the banknote was folded in two, an area of micro-printing on the banknote could be viewed through the lens. However, whereas the priority application referred only to a "printed or embossed" feature for this purpose, claim 1 of the parent application referred more generally to a "feature which can be inspected, enhanced or optically varied by the optical lens". This broader wording covered other possibilities, such as a window in the banknote that acted as a filter or a polarising window. The Appeal Board concluded that the broader claim 1 of the parent application was not entitled to the priority date, and therefore lacked novelty under Article 54(3) over the narrower embodiment disclosed in its published European divisional application. The somewhat over-ambitious claims of the parent echoed King Sennacherib's somewhat over-ambitious rule 2,700 years ago. In both cases the fate of the parent was the same.

The potential for a divisional to be cited against its own parent has come to be known as the problem of "poisonous divisionals", and is related to the problem of "toxic priority", according to which a priority application could (if allowed to publish) be cited against a later application claiming that priority, if the claims of the later application are broader than the subject matter disclosed in the priority application.

Of fundamental importance to these issues is the question of "partial priority". EPC Article 88(2) provides that, "Where appropriate, multiple priorities may be claimed for any one claim." However, a great deal of confusion has followed Opinion G2/98 of the EPO's Enlarged Board of Appeal, which stated, "The use of a generic term or formula in a claim for which multiple priorities are claimed ... is perfectly acceptable ... provided that it gives rise to the claiming of a limited number of clearly defined alternative subject matters." Following G2/98 the case law of the Boards of Appeal has not been consistent because of uncertainty about the meaning of "a limited number of clearly defined alternative subject matters."

In order to illustrate the problem, consider the following real life example from Decision T135/01. A priority application described a method of driving an electric motor in which the current was switched at a switching interval of approximately half the period of a dominant frequency of vibration of the motor. A later application, claiming priority from the earlier application, claimed a switching interval of between 1/4 and 3/4 of the same period (ie a broader range). Is such a claim entitled to multiple priorities? The EPO Appeal Board found that the claim could be split into a first notional part for a switching interval of approximately half the period, which was entitled to the priority date, and a second notional part for the complementary range of 1/4 to 3/4 of the period punctured by the range of "approximately half" the period, which was not entitled to the priority date.

However, in other cases, some EPO Boards of Appeal have held that various ranges did not correspond to a limited number of distinct alternative embodiments (as required by G2/98), with the result that the claim as a whole was denied priority. This opens the door to the problem of poisonous divisionals and toxic priority discussed above, as a claim that is not entitled to partial priority may be anticipated by another member of its own family that is entitled to priority.

The later addition of a disclaimer to the claim cannot always be relied upon to overcome the problems of poisonous divisionals and toxic priority. The EPO Guidelines for Examination (H V 4) state, "When it comes to determining whether, after the introduction of the disclaimer, the claim infringes Art. 123(2) (ie adds subject-matter) or whether it is in conformity with it, this cannot be decided solely by establishing that the disclaimed subject-matter is disclosed in the application as filed." Instead, the Guidelines state that the test is whether the skilled person would regard the remaining claimed subject matter as directly and unambiguously disclosed in the application as filed. Therefore, when filing a European application (or a PCT application intended to enter the European regional phase) that broadens the disclosure of the priority application, it is advisable to ensure that the European application contains proper support for any disclaimer that may be required if another European application in the same family were to be cited against the European application. However, in cases where the limits of the disclosure of the priority application are not clearly defined (for example, if the word "about" or "approximately" is used), the use of a disclaimer in a later application may in any case be problematic.

Fortunately, there is light at the end of the tunnel, as the issues of partial priority and of divisionals and parents being cited against each other have now been referred to the Enlarged Board of Appeal. The purpose of the Enlarged Board of Appeal is "to ensure uniform application of the law".

In the Board of Appeal Decision T 0557/13 dated 17 July 2015 (in which a parent was cited against its own divisional) the Board stated, "There continues to be a lack of convergence in the jurisprudence, which underlines the expediency of referring the matter to the Enlarged Board."

The Enlarged Board has told us that they hope to issue a decision before the summer. This could mean that European patent applications can no longer suffer same fate as King Sennacherib.