Examples compilation now returns first chars of failing code

This closes #2048. Also fixes an error in traps.

Author: JJ

doc/Language/traps.pod6

@@ -59,12 +59,12 @@ You can purposefully assign C<Nil> as a default value:
 
 =begin code
 my %h is default(Nil) = a => Nil;
-dd %h; # OUTPUT: «Hash %h = {:a(Nil)}␤»
+say %h; # OUTPUT: «Hash %h = {:a(Nil)}␤»
 =end code
 
 Or bind a value to C<Nil> if that is the result you want:
 
-=begin code
+=begin code :preamble<my @a = 1,2,3,4;>
 @a[3] := Nil;
 say @a; # OUTPUT: «[4 8 (Any) Nil]␤»
 =end code

xt/examples-compilation.t

@@ -76,11 +76,11 @@ for @examples -> $eg {
 
     # #1355 - don't like .WHAT in examples
     if ! $eg<ok-test>.contains('WHAT') && $eg<contents>.contains('.WHAT') {
-        flunk "$eg<file> chunk $eg<count>" ~ ' uses .WHAT: try .^name instead';
+        flunk "$eg<file> chunk starting with «" ~ starts-with($eg<contents>) ~ '» uses .WHAT: try .^name instead';
         next;
     }
     if ! $eg<ok-test>.contains('dd') && $eg<contents> ~~ / << 'dd' >> / {
-        flunk "$eg<file> chunk $eg<count>" ~ ' uses dd: try say instead';
+        flunk "$eg<file> chunk starting with «" ~ starts-with($eg<contents>) ~ '» uses dd: try say instead';
         next;
     }
 
@@ -104,7 +104,7 @@ for @examples -> $eg {
     $code ~= "\{}\n" if $eg<method> eq "True";
     $code ~= "\n}}";
 
-    my $msg = "$eg<file> chunk $eg<count> starts with “" ~ ($eg<contents>.lines)[0].substr(0,10).trim ~ "” compiles";
+    my $msg = "$eg<file> chunk $eg<count> starts with “" ~ starts-with($eg<contents>) ~ "” compiles";
 
     my $status;
     {
@@ -122,3 +122,7 @@ for @examples -> $eg {
         pass $msg;
     }
 }
+
+sub starts-with( Str $chunk ) {
+    ($chunk.lines)[0].substr(0,10).trim
+}