10 REM ibm: fire (model of fire with recycled nutrients)
20 CLS
30 SCREEN 1, 0: COLOR 15, 0
35 LINE (0, 0)-(319, 180), 0, BF
40 LINE (0, 0)-(319, 180), 3, B
50 g1 = 18000:  REM threshold amount of grass to turn fire on
60 n0 = 25:     REM total nutrients in soil and in grass (N+P*Q)
70 q = 1000:    REM quantity of grass / m2
80 e = 25:      REM energy source
90 k2 = .005
100 f = 16000:  REM amount consumed by fire
110 p = .001:   REM proportion of grass that is nutrients
125 IF q > g1 THEN LINE (t, 180 - g1 / 300)-(t, 180 - 6.667), 2
130 IF q > g1 THEN q = q - f: n = n0
150 PSET (t, 180 - n * 2), 3
170 PSET (t, 180 - q / 300), 1
180 q = q + n * e - k2 * q
190 n = n0 - p * q:     REM n is nutrients in soil
200 t = t + 1
210 IF t < 319 GOTO 125
215 LOCATE 2, 2: PRINT "Fire.bas"
220 SLEEP: SLEEP
230 END