If the search string is absent from the main string then
REMAIN$ returns the main string ... I had expected it to
return the null string. The statement in the documentation
that "REMAIN$ is a complement to the EXTRACT$ function" is
somewhat misleading in this case. So the identity

main$ = extract$(main$, ",") + "," + remain$(main$, ",")

is true only if main$ contains a ",". Personally I would
like to see this behavior changed in any update.