sub iterate(&$) {
   my ($callback, $envs) = @_;
                      
   while (my ($env_name,      $platforms) = each %{$envs     }) {
   while (my ($platform_name, $hosts    ) = each %{$platforms}) {
   while (my ($host_name,     $targets  ) = each %{$hosts    }) {
   while (my ($target_name,   $target   ) = each %{$targets  }) {
      my $total_capacity = $target->{total_capacity};
      my $free_capacity  = $target->{free_capacity};
      
      $callback->(
         $env_name,
         $platform_name,
         $host_name,
         $target_name,
         $total_capacity,
         $free_capacity,
      );
   }}}}             
}