= if(
  /* Do we need to add?*/
  and(
    not(
      isnull(
        ri!value
      )
    ),
    /* Yes, if the value isn't null*/
    ri!index = 1 + length(
      ri!array
    )/* and we're at the end of the list*/
    
  ),
  append(
    ri!array,
    ri!value/* Add: Append the new value to the end of the list*/
    
  ),
  if(
    /* Do we need to remove?*/
    isnull(
      ri!value
    ),
    /* Only if value is now null (value was removed)*/
    remove(
      ri!array,
      ri!index
    ),
    /* Remove: remove the list item for this field*/
    insert(
      /* Neither add nor remove, so insert at index*/
      ri!array,
      ri!value,
      ri!index
    )
  )
)