+// test: echo -e 'set (1, 2, 3, 3.11, pi, 4)\nget (4)' | calc.exe | grep '=> 3.11'
+// test: echo -e 'set (1, 2, 3, 3.11, pi, 4)\npop\npop\n5\npush\nshow' | calc.exe | grep 'stack: 1 2 3 3.11 5'
+// test: echo -e 'set (1, 2, 3, 3.11, pi, 4)\nset\nput (3, 4)\nshow' | calc.exe | grep 'stack: 0 0 4'
+// test: echo -e 'set (1, 2, 3, 3.11, pi, 4)\nlen' | calc.exe | grep '=> 6'
+// test: echo -e 'set (1, 2)\npop\npush (3)\nput (5, -1)\nlen\nshow\nget (3)\nquit' | calc.exe -n -v 3 | grep -q bye
+// test: echo -e 'put\nget\nget (1)\npop\nput (0)' | calc.exe | grep -c 'error' | xargs test 5 =