No stack space issue ... !

DECLARE SUB ADD (n)
COMMON SHARED z&

n = 20
z& = 1

CALL ADD(n)

PRINT z&

END

SUB ADD (n)

IF n < 1 THEN EXIT SUB

z& = z& + 1

CALL ADD(n - 1)
CALL ADD(n - 1)

END SUB