x = [1,2,3];
printf("%a\n", head(x));
printf("%a\n", head(x));
printf("%a\n", head(x));
printf("%a\n", head(tail(x)));
printf("%a\n", head(tail(tail(x))));
printf("%a\n", pop(x));
printf("%a\n", pop(x));
printf("%a\n", pop(x));
printf("%a\n", isempty(x));
push(x,4);
push(x,5);
push(x,6);
printf("%a\n", length(x));
printf("%a\n", pop(x));
printf("%a\n", pop(x));
printf("%a\n", pop(x));
append(x,4);
append(x,5);
append(x,6);
printf("%a\n", length(x));
printf("%a\n", pop(x));
printf("%a\n", pop(x));
printf("%a\n", pop(x));
printf("%a\n", equal([1,2,3], append([1,2],3)));
printf("%a\n", equal([], []));
printf("%a\n", equal([1], []));
printf("%a\n", equal([], [1]));
printf("%a\n", equal([], x));