spreadsheet app

[fabila]

Sorry for my English
I'm trying to make a program app based on the spreadsheet
when I try to get equations from within the program, these are evaluated correctly, but the cell It does not keep the equation.
How I can get the cell keeps its equations?

Code:


EXPORT spreadsheet ()
BEGIN
END;
START ()
BEGIN
A1:=12;
A2:=7;
B2:=A1+A2;
END;

the program is evaluated correctly, but if I change the values ​​of the equation spreadsheet is not re-evaluate