“…This revival has been both practical [8,9,14,15] (for example, at the microscopic level, in connection with the calculation of 7i-electron "ring-currents" in conjugated systems, in which the concept of spanning tree is of relevance [9]) and mathematical [4][5][6][7][10][11][12][13] (extending the classic "Matrix-Tree" theorem [2], thereby leading to devices [5][6][7][10][11][12][13] that give ever-more efficient ways of counting spanning trees). These new methods [10][11][12][13] have already been applied [14,15] to establish the "complexity" of (i.e., the number of spanning trees in) systems such as the topical buckminsterfullerene [14,15], in which the spanning-tree count is of the order of IO 20 .…”