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 <a onClick="popup('http://www.biomedcentral.com/1471-2105/13/S10/S12/mathml/M90','MathML',630,470);return false;" target="_blank" href="http://www.biomedcentral.com/1471-2105/13/S10/S12/mathml/M90">View MathML</a> shows its two shallowest degree-two nodes v1 and v2. R1 and R2 are the resulting trees of this operation. That is, <a onClick="popup('http://www.biomedcentral.com/1471-2105/13/S10/S12/mathml/M91','MathML',630,470);return false;" target="_blank" href="http://www.biomedcentral.com/1471-2105/13/S10/S12/mathml/M91">View MathML</a> where R1 = Γ(S, X, v1) and R2 = Γ(S, X, v2).

Lin et al. BMC Bioinformatics 2012 13(Suppl 10):S12   doi:10.1186/1471-2105-13-S10-S12