Each symbol a is associated with its supplement a which satisfies the equivalences a+a = i, aa = o, the latter of which means that a and a have no region in common.
If certain equivalences between volumes in different countries are stated here, it`must be plainly understood that they are only known to be approximate results, and not to give a certain basis for any theories of derivation.
If we were to say that on his view the essential step must be the establishment of identities or equivalences, we should probably be doing justice to his doctrine of numerical reasoning, but should have some difficulty in showing the application of the method to geometrical reasoning.