This is the PD Notation for an oriented link written in the form used by the computer program KnotTheory. For example, the PD notation for L4a1{0} is {{6,1,7,2},{8,3,5,4},{2,5,3,6},{4,7,1,8}}, and this becomes PD[X[6,1,7,2],X[8,3,5,4],X[2,5,3,6],X[4,7,1,8]].