Function used internally by Fpop and Fpop_w to do the backtracking and recover the best set of changes from 1 to i
retour_op(path, i)vector of length n containing the best last changes for any j in \([1, n]\). This vector is computed in the Fpop and Fpop_w using the colibri_op_c or colibri_op_weight_c function.
the last position to consider to start the backtracking.
set of optimal changes up to i.