What is the best way to get an “available index” i.e
foo1, foo2, foo3 ---> foo4
and
foo1, foo3 ---> foo2 or foo4 (but not foo3 as that already exists)
There is the $action-createtiddler with the $basetitle parameter but I don’t think this is the right tool here because no new tiddler should be created.
Thank you 
Assuming you have a list of “used” values, perhaps defined like this:
<$let used="foo1 foo3 foo5" pre="foo">
Then, to get the next index, without filling in "gaps", you can write something like:
{{{ [enlist<used>removeprefix<pre>sort[]last[]add[1]addprefix<pre>] }}}
or, to get the next index, filling in gaps, you can write something like:
{{{ [range[99999]] -[enlist<used>removeprefix<pre>] +[first[]addprefix<pre>] }}}
-e
Wonderful… worship 