[GiNaC-list] using an "undefined" function

Hi, I try to use an undefined function, such as f(x), instead of f(x)=3*x+... I know that the function depends on variable x, but not in what form. With SymbolicC++, if I define Symbolic x("x"), f[x] and g[x], the operation diff( f+g ,x ) returns diff(f,x)+diff(g,x). (the differentiation rule of sum operator) It is possible to make this with GiNaC? Thanks