Changes Iterable example, closes #2074

doc/Type/Iterable.pod6

@@ -13,14 +13,27 @@ C<Iterable> objects nested in other C<Iterable> objects (but not within scalar
 containers) flatten in certain contexts, for example when passed to a slurpy
 parameter (C<*@a>), or on explicit calls to C<flat>.
 
-Its important aspect is a method stub for C<iterator>.
+Its most important aspect is a method stub for C<iterator>.
 
-    role DNA does Iterable {
-        method iterator(){ self.comb.iterator }
-    };
+=begin code
+class DNA does Iterable {
+    has $.chain;
+    method new ($chain where { $chain ~~ /^^ <[ACGT]>+ $$ / } ) {
+        self.bless( :$chain );
+    }
 
-    my @a does DNA = 'GAATCC';
-    .say for @a; # OUTPUT: «G␤A␤A␤T␤C␤C␤»
+    method iterator(DNA:D:){ $.chain.comb.rotor(3).iterator }
+
+};
+
+my $a := DNA.new('GAATCC');
+.say for $a; # OUTPUT: «(G A A)␤(T C C)␤»
+=end code
+
+This example mixes in the Iterable role to offer a new way of iterating over
+what is essentially a string (constrained by
+L<C<where>|/type/Signature#index-entry-where_clause_(Signature)> to just the
+four DNA letters). In the last statement, C<for> actually hooks to the C<iterator> role printing the letters in groups of 3.
 
 =head1 Methods