Add supply/emit to control structures

Author: lizmat

doc/Language/control.pod6

@@ -490,6 +490,22 @@ methods that are called from within C<gather>:
 
 If values need to be mutable on the caller side, use L<take-rw|/type/Mu#routine_take-rw>.
 
+=head1 X<supply/emit|control flow,supply emit>
+
+Emits the invocant into the enclosing
+L<supply|/language/concurrency#index-entry-supply_(on-demand)>:
+
+    my $supply = supply {
+        emit $_ for "foo", 42, .5;
+    }
+    $supply.tap: {
+        say "received {.^name} ($_)";
+    }
+
+    # OUTPUT:
+    # received Str (foo)
+    # received Int (42)
+    # received Rat (0.5)
 
 =head1 X<given|control flow, given>
 X<|switch (given)>