#include <string>
#include <iostream>
 
std::string &concat( std::string &a, const std::string &b ) {
    a += b;
    return a;
}
std::string &concat( std::string &&a, const std::string &b ) {
    a += b;
    return a;
}
 
int main( void ) {
    std::string a = "abc";
    std::string b = "def";
    std::cout << concat( a, b ) << std::endl;
    std::cout << concat( a, (a + b) ) << std::endl;
    std::cout << concat( (a + b), b ) << std::endl;
    return 0;
}

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