100 REM Fibonacci sequence
110 DECLARE EXTERNAL FUNCTION Fib
120 INPUT PROMPT "Enter n for fib(n): ": N
130 PRINT Fib(N)
140 END
150 REM ***
160 EXTERNAL FUNCTION Fib(N)
170 IF N = 0 THEN
180    LET Fib = 0
190 ELSEIF N = 1 THEN
200    LET Fib = 1
210 ELSE
220    LET Prev = 0
230    LET Curr = 1
240    FOR I = 2 TO N
250       LET T = Curr
260       LET Curr = Prev + Curr
270       LET Prev = T
280    NEXT I
290    LET Fib = Curr
300 END IF
310 END FUNCTION