doc: manual: add text on the interaction of extrasymbols and dummy indices

Author: jodavies

doc/manual/variable.tex

@@ -780,6 +780,28 @@ \section{Dummy indices}
 found with which the above cases could be made to work without the penalty 
 in execution time, it will be built in in the future.
 
+One must also take care when working with extra symbols when processing
+expressions which contain dummy indices. Dummy indices which are inside
+the extra symbol definitions are not visible to {\FORM}'s renumbering
+routines, which can lead to ambiguous results. For example:
+\begin{verbatim}
+    CFunction f,g;
+    Index mu;
+
+    Local test = f(mu,mu)*g(N1_?,N1_?);
+    ArgToExtraSymbol g;
+    Sum mu;
+    Argument g;
+        FromPolynomial;
+    EndArgument;
+
+    Print;
+    .end
+
+   test =
+      f(N1_?,N1_?)*g(N1_?,N1_?);
+\end{verbatim}
+
 \section{Kronecker delta's}
 
 \label{sect-kroneckerdelta}
@@ -825,7 +847,8 @@ \section{Extra Symbols}
 needed for some new manipulations and can also be very handy for output 
 that is to be treated by for instance a FORTRAN or C compiler. The 
 FromPolynomial statement replaces the extra symbols again by their original 
-meaning.
+meaning. Care should be taken if extra symbol definitions include dummy
+indices, see \ref{sect-dummies}.
 
 % THIS EXAMPLE IS PART OF THE TESTSUITE. CHANGES HERE SHOULD BE APPLIED THERE AS
 % WELL! (Var_Extra_Symbols_1)