I now believe that things like FIRST and NEXT should be left open as variable names, so you can write things like first: first of block or next: next of group.
And it's easy to alias them, just say first: first-of/ and you'll be set, if you're working on a file or in a function where you're doing a lot of series manipulations and it's worth sacrificing the words.
Only Question Now... Is BACK the Right Term?
back of series sounds a bit too much like what TAIL does. e.g. BACK is the opposite of FRONT, not of NEXT.
Rebol 1.0 used PAST, which is the opposite of FUTURE, and I don't like it.
How about PRIOR? It's shorter than PREVIOUS, just one more character than BACK.
pos: prior of series
take prior of series