- expressions can now be read from streams; the input expression can contain

- expressions can now be read from streams; the input expression can contain
  a list of predefined symbols to be used, before reading (and overwriting)
  the expression, e.g:
    symbol x("x"), y("y");
    e = lst(x, y);
    cout << "Enter an expression containing 'x' and 'y': ";
    try {
        cin >> e;
    } catch (exception &e) {
        cerr << e.what() << endl;
    }
    cout << "You entered " << e << endl;
    cout << "de/dx = " << e.diff(x) << endl;
    cout << "de/dy = " << e.diff(y) << endl;