System alias prev_n can only appear in basic templates.

Previous edge prev_e:

• Consider the edge or node represented by a single-edge/node template shown in (a) and (b), their prev_e refers to their preceding edge;
• Consider the edges or nodes represented by a multiple-edge template shown in (c) and (d), their prev_e refers 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 true by default, while result of other conditional judgements is unpredictable.