sub _engine {
     
    trace() if $ENV{TRACE};
 
    my $self = shift;
    my $p = shift;
    my $struct = shift;
 
    my $engine
      = defined $p->{engine} ? $p->{engine} : $self->{params}{engine};
 
    if (not $engine or $engine eq ''){
        return $struct;
    }
 
    my $cref;
 
    if (not ref($engine) eq 'CODE'){
 
        # engine is a name
 
        my $engine_module = $self->{namespace} . "::Engine";
        my $compiler = $engine_module->new;
 
        # engine isn't in the dispatch table
 
        if (! $compiler->exists($engine)){
            confess "engine '$engine' is not implemented.\n";
        }
 
        eval {
            $cref = $compiler->{engines}{$engine}->();
        };
 
        # engine has bad func val in dispatch table, but key is ok
 
        if ($@){
            $@ = "\n[Devel::Examine::Subs speaking] " .
                  "dispatch table in Devel::Examine::Subs::Engine " .
                  "has a mistyped function as a value, but the key is ok\n\n"
            . $@;
            confess $@;
        }
    }
 
    if (ref($engine) eq 'CODE'){
        $cref = $engine;
    }
 
    if ($self->{params}{engine_dump}){
        my $subs = $cref->($p, $struct);
        print Dumper $subs;
        exit;
    }
 
    return $cref;
}