SUB wclear
   SET COLOR MIX(1) 0,0,0
   PLOT AREA: -10 ,-9 ; 10 ,-9 ; 10 ,-10 ; -10 ,-10
    
   SET COLOR MIX(1) 1,0,0
   PLOT AREA: 9 ,-9 ; 10 ,-9 ; 10 ,-10 ; 9 ,-10
   SET TEXT HEIGHT 0.8
    
   SET COLOR MIX(1) 0,0,0
   PLOT TEXT ,AT 9,-10 : "��"
END SUB

SUB savemouse
   if left=1 then
      let  oldleft=1
   else
      LET  oldleft=0
   end if   
   if right=1 then
      let  oldright=1
   else
      LET  oldright=0
   END IF
END SUB

SET LINE WIDTH 4
SET WINDOW -10,10,-10,10
SET POINT STYLE 3

90 CALL wclear
    
   CALL savemouse
    
100 MOUSE POLL x,y,left,right
    if left=1 then
       IF oldleft=0 THEN
          SELECT CASE INT(y)
          CASE -9,-10
             SELECT CASE INT(x)
             CASE 9,10
                SET COLOR MIX(1) 0,0,0
                CLEAR
                SET COLOR MIX(1) 0,0,0
                GOTO 90
             CASE ELSE
             END SELECT
          CASE ELSE
          END  SELECT
       else 
          SET COLOR MIX(1) 0,0,0
          PLOT LINES: oldx, oldy ; x, y
       end if
    else 
       if right=1 then
          SET COLOR MIX(1) 1,1,1
          PLOT LINES: oldx, oldy ; x, y
       end if
    end if
     
    LET oldx = x
    LET oldy = y
     
    IF left=1 THEN 
       IF y<-9 THEN
          CALL  wclear
       END IF
    ELSE
       IF right=1 THEN 
          IF y<-9 THEN CALL  wclear
       END IF
    END IF
     
    CALL savemouse
     
    goto 100
     
    PLOT LINES: x , y
200 END