allow replacing particular parameters

lib/trace.js

@@ -2,19 +2,20 @@
 var EventEmitter = require('events').EventEmitter;
 var util = require('util');
 
-function extendTrace(object, property) {
+function extendTrace(object, property, pos) {
     var old = object[property];
-    object[property] = function(x) {
+    object[property] = function() {
         var error = new Error();
         var name = object.constructor.name + '#' + property + '(' + 
             Array.prototype.slice.call(arguments).map(function(el) {
                 return util.inspect(el, false, 0);
             }).join(', ') + ')';
 
-        var last = arguments.length - 1;
-        var cb = arguments[last];
-        if (typeof arguments[last] === 'function') {
-            arguments[last] = function replacement() {
+        if (typeof pos === 'undefined') pos = -1;
+        if (pos < 0) pos += arguments.length;
+        var cb = arguments[pos];
+        if (typeof arguments[pos] === 'function') {
+            arguments[pos] = function replacement() {
                 try {
                     return cb.apply(this, arguments);
                 } catch (err) {