std::string gettoknth(std::string &str, int n) {
std::string::size_type s = 0, e = 0;
if (!findtoknth(str, n, s, e))
return "";
return str.substr(s, e - s);
}
bool changetoknth(std::string &str, int n, const char *with, bool insert = false, bool nonzero = false)
{
std::string::size_type s = 0, e = 0;
if (!findtoknth(str, n, s, e))
return false;
if (nonzero && str.substr(s,e-s) == "0")
return true; // not an error
if (!insert)
str.replace(s, e-s, with);
else
str.insert(s, with);
return true;
}