[Ferguson, Neale]

I am a C++ dummy so can any one tell me what I should look for to fix the
following. The code says:

void CallStack::Output(ostream &os)
{

    total_time = data.total_time();

    for (ProfileData::const_arc_iterator i = data.begin_roots();
         i != data.end_roots(); ++i)
    {
        os << root << ' ' << get_func_time(*i) << '\n';
        output_node(os, *i, 1);
    }
}

The compiler bitches:

callstack.C: In member function `void CallStack::Output(std::ostream&)':
callstack.C:83: no match for `ProfileData::const_arc_iterator& !=
   ProfileData::const_arc_iterator' operator