There are two sorts of multi-value attribute types" List and Set. Lists allow dupes and do not, of themselves alters their sort order—though Action code offers means to do so deliberately. Sets de-dupe and generally retain their last used order but if in doubt, pass the data to a list before work.
Thus @eastgate’s solution works.
ay() is zero-based so
.at(0) is the first item in the list. Here,
might be more easily understood as:
but happily, Tinderbox lets us do all that in one operation. So in either method above, if $MyList is “and:bee;cow” the result is “bee;cow” as “ant” is the first list item (
.at(0)) of the list at the start of the process.