Definition 2.1
A Circular orderC(T) of set of sequences
is any
tour through a tree T(S) where each edge is traversed exactly twice,
and each leaf is visited once.

When a tree is traversed in a circular order, that is, from leaf A
to B, from B to C, from C to D, from D to E and then back from E
to leaf A (Figure 2), all edges are counted exactly
twice. A circular order is the
shortest possible tour through a tree where each leaf is visited
once (shortest means smallest sum of edge
lengths) (see Figure 2) [15].