Add simple trap example for Captures

Author: skids

lib/Language/traps.pod

@@ -109,7 +109,7 @@ inside C<BUIDLALL>, but not inside C<BUILD>).
 
 =head2 Whitespace in Regexes does not match literally
 
-    $ perl6-e "say 'a b' ~~ /a b/"
+    $ perl6 -e "say 'a b' ~~ /a b/"
     False
 
 Whitespace in regexes is, by default, considered an optional filler without
@@ -123,5 +123,24 @@ Ways to match whitespace:
 =item C<\h>, C<\v> for horizontal, vertical whitespace
 =item with C<m:s/a b/> or C<m:sigspace/a b/>, the blank in the regexes matches arbitrary whitespace
 
+=head1 Captures
 
+=head2 Variables versus values/expressions in a Capture
+
+Beginners might expect a variable in a C<Capture> to supply its current
+value when that C<Capture> is later used.  For example:
+
+    $perl6 -e 'my $a = 2; say($a, ++$a)'
+    33
+
+Here the C<Capture> contained the B<variable> C<$a> and the B<value> of the result
+of the expression C<++$a>.  Since the C<Capture> must be reified before C<&say> can
+use it, the C<++$a> may happen before C<&say> gets C<$a>, and so it may already
+be incremented.
+
+Instead, use an expression that produces a value when you want a value.
+
+    $perl6 -e 'my $a = 2; say(+$a, ++$a)'
+    23
+    
 =end pod