[GiNaC-list] Undefine the predefined numeric consant 'I'?

Is there an easy way to undefine the predefined numeric value 'I'? If I define a symbol 'I', use it in an expression, and then output that expression to cout after modifying stream with csrc, 'I' is converted to 'std::complex<double>(0.0,-1.0)'. I'd like the output to be just 'I'. Warren