controlling backtrace with env var RAKUDO_BACKTRACE

gistfile1.pl

--- a/src/core/Exception.pm
+++ b/src/core/Exception.pm
@@ -69,10 +69,15 @@ do {
                 $err.print: $e.gist;
                 $err.print: "\n";
             }
-            elsif is_runtime($ex.backtrace) {
+            elsif %*ENV<RAKUDO_BACKTRACE> | is_runtime($ex.backtrace) {
                 $err.print: $e;
                 $err.print: "\n";
-                $err.print: Backtrace.new($e);
+                my $b :=  Backtrace.new($e);
+                given %*ENV<RAKUDO_BACKTRACE> {
+                  when 'concise'  { $err.print: $b.concise  }
+                  when 'full'     { $err.print: $b.full     }
+                  when *          { $err.print: $b.Str      }
+                }
             }
             else {
                 $err.print: "===SORRY!===\n";