Before revision:  In addition to allowing this flexibility at the slow time scale  , the proposed methods utilize a so-called `infinitesimal' formulation for the fast time scale through definition of a sequence of modified ``fast'' initial-value problems, that may be solved using any viable solver .
After revision:  In addition to allowing this flexibility at the slow time scale flexibility , the proposed methods utilize a so-called `infinitesimal' formulation for the fast time scale through definition of a sequence of modified ``fast'' initial-value problems, that may be solved using any viable solver .