Sum function

DECLARE FUNCTION sum (z%)
COMMON SHARED s%

Z% = 3

s% = sum(z%)

PRINT s%

END

FUNCTION sum (z%)

IF z% = 0 THEN EXIT FUNCTION

sum = sum(z% - 1) + z%

END FUNCTION

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This is working since the function is like f(n) = f(n - 1) + Variable, but what about a function like f(n) depends on 2 previous states...

n - 1 and n - 2.