doc submethod BUILD

Author: gfldex

doc/Language/typesystem.pod6

@@ -42,6 +42,29 @@ such they can not be altered from outside the class they are defined in.
 
 =head3 Methods
 
+=head4 Submethod BUILD
+
+The submethod C<BUILD> is called by the submethod C<BUILDALL> defined in
+L</type/Mu>, which in turn is called by C<.bless>. It is meant to set private
+and public attributes of a class and receives all names attributes passed into
+C<.bless>. Since it is called by C<BUILDALL> it is called by the default
+constructor C<.new> defined in C<Mu>.
+
+    class C {
+        has $.attr;
+        submethod BUILD (:$attr = 42) {
+            $!attr = $attr
+        };¶
+        multi method new($positional) {
+            self.bless(:attr($positional), |%_)
+       }
+    };
+    
+    C.new.say; C.new('answer').say;
+    # OUTPUT:
+    # C.new(attr => 42)
+    # C.new(attr => "answer")
+
 =head4 Fallback method
 X<|FALLBACK (method)>