view: add head/tail accessors that return truncated subviews

view.hpp

@@ -308,6 +308,24 @@ namespace util {
         }
 
 
+        //---------------------------------------------------------------------
+        template <typename IndexT>
+        constexpr auto
+        head (IndexT idx)
+        {
+            return std::get<0> (split (idx));
+        }
+
+
+        //---------------------------------------------------------------------
+        template <typename IndexT>
+        constexpr auto
+        tail (IndexT idx)
+        {
+            return std::get<1> (split (idx));
+        }
+
+
         //---------------------------------------------------------------------
         template <typename IndexT>
         [[nodiscard]] constexpr auto