doc is trait in typesystem

doc/Language/typesystem.pod

@@ -57,7 +57,34 @@ be searched use a C<HyperWhatever>.
 
 =head3 trait C<is>
 
-TODO
+Defined as:
+
+    multi sub trait_mod:<is>(Mu:U $child, Mu:U $parent)
+
+The trait X<C<is>|is (inheritance)> accepts a type object to be
+added as a parent class of a class in it's definition. To allow multiple
+inheritance the trait can be applied more then once. Adding parents to a class
+will import their methods into the target class. If the same method name occurs
+in multiple parents, the first added parent will win.
+
+If no C<is> trait is provided the default of L<C<Any>|/type/Any)> will be used
+as a parent class. This forces all Perl 6 objects to have the same set of basic
+methods to provide an interface for introspection and coercion to basic types.
+
+    class A { 
+        multi method from-a(){ 'A::from-a' }
+    }
+    dd A.new.^parents(:all);
+    # OUTPUT«(Any, Mu)␤»
+
+    class B {
+        method from-b(){ 'B::from-b ' }
+        multi method from-a(){ 'B::from-A' }
+    }
+
+    class C is A is B {}
+    dd C.new.from-a();
+    # OUTPUT«"A::from-a"␤»
 
 =head3 trait C<is required>