Resolution:
## 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 v_{1} and v_{2}. R_{1} and R_{2} are the resulting trees of this operation. That is, where R_{1} = Γ(S, X, v_{1}) and R_{2} = Γ(S, X, v_{2}).
Lin |