sub warn {
  my @messages = @_;
  unless (@messages) {
    @messages = "Warning: something's wrong";
  }
  if ($messages[-1] !~ /\n$/) {
    my ($package, $file, $line) = caller();
    $messages[-1] .= " at $file line $line\n";
  }
  if ($SIG{__WARN__}) {
    $SIG{__WARN__}->(@messages);
  }
  else {
    print STDERR @messages;
  }
}