To run:
Start up.
Test 123.
Test 12345.
Test 1234567.
Test 987654321.
Test 10001.
Test -10001.
Test -123.
Test -100.
Test 100.
Test -12345.
Test 1.
Test 2.
Test -1.
Test -10.
Test 2002.
Test -2002.
Test 0.
Wait for the escape key.
Shut down.

To get the middle three digits of a number giving a string:
Privatize the number.
De-sign the number.
Convert the number to the string.
If the string's length is less than 3, put "Number has fewer than three digits" into the string; exit.
If the string's length is even, put "Number has no middle" into the string; exit.
Put the string's length divided by 2 into a midpoint number.
Slap a substring on the string.
Put the substring's first plus the midpoint minus 1 into the substring's first.
Put the substring's first plus 2 into the substring's last.
Put the substring into the string.

To test a number:
Get the middle three digits of the number giving a string.
Write "" then the number then "  ->  " then the string on the console.