sub _length{
  my $a = shift;
  my $dim = shift;
  if ( ref( $a ) =~ /ARRAY/ ){
    print "\t" x $dim;
    print "dim $dim -> ".(scalar @$a)." elements";
    print "\n" if $dim == 0;
    print "\n";
    _length( $_, $dim + 1) for ( @$a );
  }
}