reductions: revert back to associative binary ops

content/programming_model_overview.tex

@@ -157,7 +157,7 @@
       of concatenated symmetric objects contributed by each of the \acp{PE} in
       another symmetric data object.
   \item \OPR{Reduction}: All \acp{PE} participating in the routine get the result
-      of a binary operation over elements of the specified symmetric
+      of an associative binary routine over elements of the specified symmetric
       data object on another symmetric data object.
   \item \OPR{All-to-All}: All \acp{PE} participating in the routine exchange
       a fixed amount of contiguous or strided data with all other participating