Writable does not require Copyable, fixes #240

iterators.tex

@@ -1098,7 +1098,7 @@
 \begin{codeblock}
   template <class Out, class T>
   concept bool Writable() {
-    return Semiregular<Out>() &&
+    return Movable<Out>() && DefaultConstructible<Out>() &&
       requires(Out o, T&& t) {
         *o = std::forward<T>(t); // not required to be equality preserving
       };