*{'deep_walk'} = build_deep_sub();
  *{'deep_map'} = build_deep_sub( init => 'my $test = shift;', map => 'map $test->($_),' );
  *{'deep_grep'} = build_deep_sub( init => 'my $test = shift;', map => 'grep $test->($_),' );
  *{'deep_grep_refs'} = build_deep_sub( init => 'my $test = shift;', map => 'grep $test->($_),', guard => 'ref($_)' );
  *{'deep_grep_type'} = build_deep_sub( init => 'my $type = shift;', map => 'grep isa( $_, $type ), ', guard => 'ref($_)' );