We doubt that Brian is a swan. We doubt that Lily is a swan. It is highly unlikely that Julius is a frog.
We believe that either Julius is a frog, or Lily is a swan.
It is probably not the case that either Julius is a frog, or Lily is a swan.
We believe that either Julius is a frog, or Lily is a swan.
Either Julius is a frog, or Lily is a swan.