The VinClu clustering map

AB->arb The VinClu clustering map is a 3->2 clustering map rather similar to that of the Arclus jet clustering algorithm. Its principal advantage is that it contains the exact inverses of the Vincia kinematics mappings, and hence it can be used to "undo" branchings and/or reconstruct the exact intermediate shower states from a given momentum configuration. The clustering is invoked using: Here iMap is the index number of the kinematics map to be used (the numbering follows VinciaShower:kineMapType), pIn is a vector of arbitrary size containing the state of particles to be clustered, and the indices a, r, b represent the 3 particles to be clustered in the backwards reconstruction of AB->arb. pClu is initially set equal to p. The reconstructed momenta of the parents, A and B, are then stored in pClu[a] and pClu[b], respectively, and finally pClu[r] is erased. (Note that this last step may cause the indices of A and/or B in pClu to be different from those of a and b in p. For the most common case, a color-ordered clustering with r=a+1 and b=a+2, the indices of A and B after the clustering will be A=a and B=A+1.)