Figure 3.

Example of the shallowest regrouping operation. Example of the shallowest regrouping operation of S by X where X = {a, c, d}. The intermediate tree shows its two shallowest degree-two nodes v₁ and v₂. R₁ and R₂ are the resulting trees of this operation. That is, where R₁ = Γ(S, X, v₁) and R₂ = Γ(S, X, v₂).