-ex operator+(ex const & lh, ex const & rh);
-ex operator-(ex const & lh, ex const & rh);
-ex operator*(ex const & lh, ex const & rh);
-ex operator/(ex const & lh, ex const & rh);
-ex operator%(ex const & lh, ex const & rh); // non-commutative multiplication
-
-/*
-
-// binary arithmetic operators ex with numeric
-ex operator+(ex const & lh, numeric const & rh);
-ex operator-(ex const & lh, numeric const & rh);
-ex operator*(ex const & lh, numeric const & rh);
-ex operator/(ex const & lh, numeric const & rh);
-ex operator%(ex const & lh, numeric const & rh); // non-commutative multiplication
-
-// binary arithmetic operators numeric with ex
-ex operator+(numeric const & lh, ex const & rh);
-ex operator-(numeric const & lh, ex const & rh);
-ex operator*(numeric const & lh, ex const & rh);
-ex operator/(numeric const & lh, ex const & rh);
-ex operator%(numeric const & lh, ex const & rh); // non-commutative multiplication
-
-*/
+const ex operator+(const ex & lh, const ex & rh);
+const ex operator-(const ex & lh, const ex & rh);
+const ex operator*(const ex & lh, const ex & rh);
+const ex operator/(const ex & lh, const ex & rh);