Clearing certain sprites using an array?

I'm currently making a game with many mini-games. One of the games is a shooting game of sorts where your ship shoots autonomously. Here's the code when the ship shoots: [Every 60 ticks, create a bullet sprite at the ship's position and store it's ID in an array]

IF (_TIME - TIME) MOD (60/1+GAME*3)) == 0 THEN
 SPSET 3 OUT IX
 PUSH BULLETS, IX
 SPHOME IX,1,1
 SPOFS 3 OUT _SHIPX,
 SPOFS IX,_SHIPX,SH-16
 SPCOL IX
END

Now, it makes sense to clear the sprite and free up memory when a sprite goes off-screen, right? Unfortunately, that's where I run into a problem: [This code includes moving the bullet as well detecting whether or not it goes off-screen]

FOR I=0 TO LEN(BULLETS)-1
 SPOFS BULLETS[I] OUT _BX,_BY
 SPANIM BULLETS[I],"XY",1,_BX,_BY-1
 IF _BY < 0 THEN
  SPCLR BULLETS[I]
  SHIFT(BULLETS)
  BREAK
 ENDIF
END

The problem with this code, however, is the SPCLR and SHIFT. I understand the SPCLR messing up, since when it clears the sprite and goes through the FOR loop again to move the cleared sprite, it won't exist. But why is SHIFT an illegal function? I mean, theoretically is should: 1. Clear the bullet 2. Remove the ID from the array Is there anything I could do to fix this code?

I believe that to use functions like SHIFT() and POP() you need to store the result you get from these functions. Like

A=SHIFT(BULLETS)

Then you just don't use A.

you could also create a function to "eat" the return value of a function

DEF EAT SAND:END

EAT SHIFT(BULLETS)

In SB, all functions with parentheses have a return value.