*~ = 0, *y*~*t*~ = *y*~*t-0*~ + 0 if *i* reverts *j* and *y*~*t*~ = *y*~*t-0*~ − 0 if *j* reverts *i* at time *t*; the labels *i* and *j* are assigned so that *y* \>= 0 for the majority of the *ij* interaction time