x = [1,2,3];
y = [4,5,6];
printf("%a\n", concat(x,y));
printf("%a\n", x);
printf("%a\n", y);


x = [];
y = [4,5,6];
printf("%a\n", concat(x,y));
printf("%a\n", x);
printf("%a\n", y);


x = [1,2,3];
y = [];
printf("%a\n", concat(x,y));
printf("%a\n", x);
printf("%a\n", y);


x = [];
y = [];
printf("%a\n", concat(x,y));
printf("%a\n", x);
printf("%a\n", y);