sub as_hex
  {
  # return as hex string, with prefixed 0x
  my $x = shift; $x = $class->new($x) if !ref($x);

  return $x->bstr() if $x->{sign} !~ /^[+-]$/;    # inf, nan etc
  return '0x0' if $x->is_zero();

  my $es = ''; my $s = '';
  $s = $x->{sign} if $x->{sign} eq '-';
  if ($CALC->can('_as_hex'))
    {
    $es = ${$CALC->_as_hex($x->{value})};
    }
  else
    {
    my $x1 = $x->copy()->babs(); my ($xr,$x10000,$h);
    if ($] >= 5.006)
      {
      $x10000 = Math::BigInt->new (0x10000); $h = 'h4';
      }
    else
      {
      $x10000 = Math::BigInt->new (0x1000); $h = 'h3';
      }
    while (!$x1->is_zero())
      {
      ($x1, $xr) = bdiv($x1,$x10000);
      $es .= unpack($h,pack('v',$xr->numify()));
      }
    $es = reverse $es;
    $es =~ s/^[0]+//;     # strip leading zeros
    $s .= '0x';
    }
  $s . $es;
  }
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