I have cheated twice here; once because of my brain refusing to solve the initial "what-happens-when-there-are-fewer-than-two-items-in-the-list" problem; and the second time , to work round an issue I was having get the expression language to recognize my data type....
Was performing a String Concatenation, rather than a arithmetic addition is down to this:
If I defined a Page on 'Page and Classes' - but did NOT specify a Class; the Expression was (apparently) assuming that the embedded Property was of type text.
This is weird - since it *did* correctly assign a 'pxObjClass' to the page at (designer studio) runtime; and correctly resolved the property .Accumulator with the correct type.
In fact: (aside from the subsequent validation failures; "page-messages"); the pages (in XML format) look identical....(attaching for reference).
Anyway: either of these three things eliminates this issue:
1. Explicitly identify the Class of the Page "Results"
2. Remove the Definition of "Results" and the leave all Step Page Blank.
3. Remove the Definition of "Results" and enter "Primary" in the Step Pages.
Additionally: I have remove the JUMP from my loop - it is not needed; but I still found I needed a 'shadow' variable for the automatic position index "<current>" - since I wasn't able to get the following expression to save: