how to construct a ex in GiNaC

I read the GiNaC code ,found a question. Under GiNaC namespace ,the under code works well. //***this is my code: ............ #include "ginac/ginac.h" using namespace GiNaC; ............ int main(void) { symbol a("a"),b("b"); ex ex0(a+b); cout<<"ex0 = "<<ex0<<endl; return 0; } // end the code but i find nothing about method ex(symbol) in GiNaC. how does the above statement ex ex0(a) work? help me !! thanks! best regards, Boqun Cheng 2003.10.17