ulong code = 0;

{ code = HTTP_200_OK;                } : (code !& ...);

{ code = HTTP_301_MOVED_PERMANENTLY; } : (code !& ...);
{ code = HTTP_302_MOVED_TEMPORARY;   } : (code !& ...);

{ code = HTTP_403_FORBIDDEN;         } : (code !& ...);
{ code = HTTP_404_NOT_FOUND;         } : (code !& ...);
{ code = HTTP_410_GONE;              } : (code !& ...);

if (code)
{
  set_header (http_header, code);
  ...
}