10 DIM A$(20,20)
 20 LET A$(10,10)="1"
 30 FOR Y=42 TO 23 STEP -1
 40 FOR X=0 TO 19
 50 PLOT X,Y
 60 NEXT X
 70 NEXT Y
 80 UNPLOT 9,33
 90 FOR I=1 TO 80
100 LET X=INT (RND*18)+2
110 LET Y=INT (RND*18)+2
120 IF A$(X,Y)="1" THEN GOTO 100
130 UNPLOT X-1,43-Y
140 IF A$(X+1,Y+1)="1" OR A$(X+1,Y)="1" OR A$(X+1,Y-1)="1" OR A$(X,Y+1)="1" OR A$(X,Y-1)="1" OR A$(X-1,Y+1)="1" OR A$(X-1,Y)="1" OR A$(X-1,Y-1)="1" THEN GOTO 230
150 PLOT X-1,43-Y
160 LET X=X+INT (RND*3)-1
170 LET Y=Y+INT (RND*3)-1
180 IF X=1 THEN LET X=19
190 IF X=20 THEN LET X=2
200 IF Y=1 THEN LET Y=19
210 IF Y=20 THEN LET Y=2
220 GOTO 130
230 LET A$(X,Y)="1"
240 NEXT I