Continues with #2069, with iterating constructs

Author: JJ

doc/Language/iterating.pod6

@@ -95,11 +95,31 @@ build an iterator type  to be returned, such as we did in the previous example;
 however, this example shows the minimal code needed to build a class that
 fulfills the two iter(ator|able) roles.
 
-
-
 =head1 How to iterate: contextualizing and topic variables
 
-TBD
+C<for> and other loops place the item produced in every iteration into the
+L<topic variable C<$_>|/language/variables#index-entry-topic_variable>, or
+capture them into the variables that are declared along with the block. These
+variables can be directly used inside the loop, without needing to declare them,
+by using the
+L<C<^> twigil|https://docs.perl6.org/syntax/$CIRCUMFLEX_ACCENT#(Traps_to_avoid)_twigil_^>.
+
+Implicit iteration occurs when using the L<sequence operator|/language/operators#index-entry-..._operators>.
+
+    say 1,1,1, { $^a²+2*$^b+$^c } … * > 300; # OUTPUT: «(1 1 1 4 7 16 46 127 475)
+
+The generating block is being run once while the condition to finish the
+sequence, in this case the term being bigger than 300, is not met. This has the
+side effect of running a loop, but also creating a list that is output.
+
+This can be done more systematically through the use of the L<C<gather/take>
+blocks|/syntax/gather take>, which are a different kind of iterating construct
+that instead of running in sink context, returns an item every iteration. This
+<Advent Calendar
+tutorial|https://perl6advent.wordpress.com/2009/12/23/day-23-lazy-fruits-from-the-gather-of-eden/>
+explains use cases for this kind of loops; in fact, gather is not so much a
+looping construct, but a statement prefix that collects the items produced by
+C<take> and creates a list out of them.
 
 =head1 C<Classic> loops and why we do not like them