di erential algebra. Note e.g. that in our algebra an element like n123 is admissible (i.e. non-zero) if n1; n2 and n2; n3 are connected. n123 may however arise from a di erentiation process (i.e. from an insertion) like du(n13) with n1; n3 not(!) connected. This is exactly the situation discussed above: n13 2 I but du(n13) = 2 I (88) Dividing now by I 0… (More)