  When from two assertibles (‘If the 1st and the 2nd, the 3rd’ and ‘But not the 3rd’) a third follows (‘Not: both the 1st and the 2nd’—this follows by a second indemonstrable) and from the third and an external one (‘The 1st’) another follows (‘Not: the 2nd’—this follows by a third indemonstrable), then this other (‘Not: the 2nd’) also follows from the two assertibles and the external one.