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 $S^{\prime }=S(\overline{X)}$ shows its two shallowest degree-two nodes v₁ and v₂. R₁ and R₂ are the resulting trees of this operation. That is, $\hat{\Gamma }\left(S,X\right)=\left\{R1,R2\right\}$ where R₁ = Γ(S, X, v₁) and R₂ = Γ(S, X, v₂).