740 REM "VIRUS" TYPE THING
  750 X=RND(39):Y=RND(29)
  760 IF FNAT(X,Y)=" "THEN GOTO 750
  770 IF Y>=30 THEN GOTO 750
  780 IF FNAT(X,Y+1)<>" "THEN Y=Y+1:GOTO 770
  790 Q$=FNAT(X,Y)
  800 PRINTTAB(X,Y);" ";TAB(X,Y+1)Q$;
  810 IF Y=31 THEN 750
  820 Y=Y+1
  830 IF FNAT(X,Y)<>" "THEN 780
  840 DEFFNAT(Q,P):PRINTTAB(Q,P);:A%=135:=CHR$((USR(&FFF4) AND &FF00)DIV&100) 
  850 RUN