Some initial documentation for TWEAK

doc/Language/objects.pod6

@@ -442,6 +442,10 @@ name. In either case, if neither C<BUILD> nor the default mechanism has
 initialized the attribute, default values are applied (the C<2 * $!x> in the
 example above).
 
+After the C<BUILD> methods have been called, methods named C<TWEAK> are
+called, if they exist, again with all the named arguments that were passed
+to C<new>.
+
 Due to the nature of the default behavior of C<BUILDALL> and custom C<BUILD>
 submethods, named arguments to the constructor C<new> derived from C<Mu> can
 correspond directly to public attributes of any of the classes in the method
@@ -521,7 +525,21 @@ class MyClass {
 }
 =end code
 
-=comment TODO: better example for BUILDALL overriding
+If all you need to do is checking things after the actual construction,
+or modify attributes after the object construction, writing a submethod
+C<TWEAK> is a good approach:
+
+=begin code
+class RectangleWithCachedArea {
+    has ($.x1, $.x2, $.y1, $.y2);
+    has $.area;
+    submethod TWEAK() {
+        $!area = abs( ($!x2 - $!x1) * ( $!y2 - $!y1) );
+    }
+}
+
+say RectangleWithCachedArea.new( x2 => 5, x1 => 1, y2 => 1, y1 => 0).area;
+=end code
 
 =head2 Object Cloning