public static MathObject Substitute(this MathObject obj, MathObject a, int b) =>
 obj.Substitute(a, new Integer(b));
 public static MathObject Substitute(this MathObject obj, MathObject a, double b) =>
 obj.Substitute(a, new DoubleFloat(b));
 public static MathObject SubstituteEq(this MathObject obj, Equation eq) =>
 obj.Substitute(eq.a, eq.b);