Figure 6

Cores will in general have 2-arcs. The structure δ (lhs) is mapped into its core c(δ) (rhs). Clearly d has arc-length ≥ 4 and as a consequence of the collapse of the stack ((1, j + 2), (2, j + 1), (i, j)) into the arc (i, j), c(δ) contains the arc (i, j), which is, after relabeling, a 2-arc.