Sometimes is it useful to match a function parameter in an unknown position and then match the arguments at call sites that correspond to that parameter. The following semantic patch illustrates how to do this:

@r@
identifier fn, x;
parameter list[n] P;
@@

fn(P, long x,...) { ... }

@@
identifier r.fn;
expression list[r.n] E;
@@

fn(E,
-    atoi
+    atol
         (...),...)

It is of course possible to go the other way, and match first an argument and then the parameter in the corresponding position, or any other combination.