A Smart Sequence needed

I know how to make a simple sequence, that will start at a value, and
increment that value. What I am needing is something like this however:

say you have a relationship such as:

1 john
2 amy
3 jim
4 sue
5 mike

ok, and a seqence will cause the next one to be 6 right? And this is fine
for me, except I want if "jim" for example is gone, he creates a hole
there, a gap. I want the gap to be filled in. So instead of a "next
available" sequence, I want a "first availabe":

1 john
2 amy
4 sue
5 mike

I want a sequence thats smart enough to pick 3 as the next "slot", since
that is the first available. If this capibility is not possible, please
let me know. It's probably quite simple, but I don't know the answer
after reading create_sequence, so I am asking it here. Thank you for the
help.

Brian

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