double tower_height(const Tower& t){
Tower::const_iterator it;
double res = 0;
for(it=t.begin(); it!=t.end(); it++){
res += it->height();
}
return res;
}
void show_tower(const Tower& t){
Tower::const_iterator it;
for(it=t.begin(); it!=t.end(); it++){
it->show();
}
}