.

Here's a simple program, probably could be simpler and its quite messy, but it does the job:

SCREEN 13
x = 1
LOCATE 1, x: PRINT "a"
space = .04
1 time1 = TIMER
2 IF time1 + space < TIMER THEN
LOCATE 1, x: PRINT " "
LET x = x + 1
IF x = 40 THEN END
LOCATE 1, x: PRINT "a"
GOTO 1
END IF
GOTO 2