[GiNaC-list] let_op(size_t ) problem

The following code: symbol x("x"); symbol y("y"); ex test1 = x + y; for ( size_t i = 0 ; i != test1.nops() ; i++ ) { try { cout << test1.let_op(i) << " " ; } catch(std::range_error & ba) { cout << "i = " << i << endl; cout << ba.what() << endl; } } This results: i = 0 basic::let_op(): add has no operands i = 1 basic::let_op(): add has no operands Using op() runs ok but I want change subexpressions. I am using GiNaC 1.3.3 on Mandriva 2006.0. Thanks.