Elementary aufbau unit

The three important types of addition (i), (ii) and (iii) have been described by Dias [16, 32-38] as an attachment of the unit (i) C4H2, (ii) C3H or (iii) C2, respectively. These are the aufbau units which Dias [32, 33] referred to as the elementary aufbau units. 

The fundamental aufbau principle is illustrated in Fig. 4. Here the aufbau units which come into operation are exactly the elementary aufbau units. The corresponding attachments of (i) C4H2, (ii) C3H and (iii) C2 are illustrated in Fig. 5 and pertain to the first example in each row (i)—(iii) of Fig. 4. But notice that the three types (a)-(c) specified in the Principles above correspond to the additions (ii), (i) and (iii) in that order. The formulation was chosen to be that way for several reasons, mainly because the type (a) is often sufficient for the aufbau, and because of our concern about extreme-left benzenoids [8] (see below). The last paragraph of the Principles gives rise to some general rules as specified in the following. 

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