const ex & operator-=(ex & lh, const ex & rh);
const ex & operator*=(ex & lh, const ex & rh);
const ex & operator/=(ex & lh, const ex & rh);
const ex & operator-=(ex & lh, const ex & rh);
const ex & operator*=(ex & lh, const ex & rh);
const ex & operator/=(ex & lh, const ex & rh);