Project Calculator (No lua)

So many things we can do in Principia.
Add addition. Add keylistener.

Now features:
Keypad input, 8-digit display, clearing, zero disable,
Key sound, main memory full detection,
Delete last digit,
Addition calculation
(press Enter to save the first number, enter the second number and press +)

Key bindings:
0-9: input number
Enter: enter
Backspace: DEL
C: Clear
A: +

I will keep update this project.