prev_n can only appear in basic templates.
- Consider the edge or node represented by a single-edge/node template shown in (a) and (b), their
prev_erefers to their preceding edge;
- Consider the edges or nodes represented by a multiple-edge template shown in (c) and (d), their
prev_erefers to their respective preceding edge.
The red arrow in the image on the right above points to an edge that doesn't exist, conditional judgements
<=related to that "edge" will return
trueby default, while result of other conditional judgements is unpredictable.