fix bug in repeated function calls

ast/token.go

@@ -48,6 +48,18 @@ func (t *Token) Append(arg *Token) {
     }
 }
 
+/* Shallow Copy
+ * in case of a LIST,
+ * inner will point to the same list.
+ */
+func (t *Token) Copy() *Token {
+    return &Token{
+        Tag: t.Tag,
+        inner: t.inner,
+        Next: t.Next,
+    }
+}
+
 /* Print function which is better suited for repl.
  * This one prints the SEXPRs as one would write them.
  * Does not evaluate tokens.