Improve documents on set ops

Author: tisonkun

doc/Language/list.pod6

@@ -94,9 +94,7 @@ One of the ways C<@>-sigiled variables act like lists is by always supporting
 L<positional subscripting|/language/subscripts>. Anything bound to a C<@>-sigiled
 value must support the L<Positional|/type/Positional> role which guarantees this:
 
-    =begin code
     my @a := 1; # Type check failed in binding; expected Positional but got Int
-    =end code
 
 =head1 Reset a List Container
 
@@ -145,14 +143,14 @@ not provide the full interface of C<Positional>, so an C<@>-sigiled variable
 may B<not> be bound to a C<Seq>.
 
     my @s := Seq.new(<a b c>); CATCH { default { say .^name, ' ', .Str } }
-    # OUTPUT«Type check failed in binding to $iter; expected Iterator but got List ($("a", "b", "c"))␤  in block <unit> at <tmp> line 1␤␤»
+    # OUTPUT «Type check failed in binding to $iter; expected Iterator but got List ($("a", "b", "c"))␤  in block <unit> at <tmp> line 1␤␤»
 
 This is because the C<Seq> does not keep values around after you have used them.
 This is useful behavior if you have a very long sequence, as you may want to
 throw values away after using them, so that your program does not fill up memory.
 For example, when processing a file of a million lines:
 
-    =begin code :skip-test
+    =begin code :preamble<sub do-something-with($x){ }>
     for 'filename'.IO.lines -> $line {
         do-something-with($line);
     }