Index arguments from 0 in with-copy

Indexing from zero is consistent with the rest of Carp's indexing
behavior, so this should make the function more predictable. I've also
added a note to the docs about this.

core/Introspect.carp

@@ -129,8 +129,8 @@
 
   (doc with-copy
     "Returns a reference to an anonymous 'proxied' variant of `function` in
-  which the argument in position `arg` is *copied* from a reference before being
-  passed to `function`:
+  which the argument in position `arg` (indexed from 0) is *copied* from a
+  reference before being passed to `function`:
 
   ```
   ;; Array.reduce expects a function that takes a *ref* in its second argument:
@@ -161,11 +161,14 @@
   (defmacro with-copy [function arg]
     ;; The calls to `eval` around `function` are necessary to ensure we can execute arity.
     (let [arg-arr (Dynamic.unreduce inc 0 (Introspect.arity (eval function)) (array))
+         ;; increment arg by 1 to simulate indexing from 0--since the
+         ;; functions we rely on here return counts
+          pos (+ arg 1)
           local-names (list-to-array-internal (map gensym-local (map Symbol.from arg-arr)) [])
-          target (gensym-local (Symbol.from arg))
+          target (gensym-local (Symbol.from pos))
           prox (list 'copy target)
           call (cons function (map (fn [x] (if (= target x) prox x)) local-names))]
-      (if (> arg (Introspect.arity (eval function)))
-          (macro-error "proxy error: the specified argument position is greater than the given function's arity.")
+      (if (> pos (Introspect.arity (eval function)))
+          (macro-error "with-copy error: the specified argument position is greater than the given function's arity.")
           (list 'ref (list 'fn local-names call)))))
 )

test/introspect.carp

@@ -55,6 +55,6 @@
     "arity works as expected")
   (assert-equal test
     6
-    (Array.reduce (Introspect.with-copy add 2) 0 &[1 2 3])
+    (Array.reduce (Introspect.with-copy add 1) 0 &[1 2 3])
     "with-copy works as expected")
 )