what's a du'u?

[Thomas Jack <thomasjack@gmail.com>]

Suppose {la .tom. du la .tomas.} is true.

What if any is the difference in meaning, then, between {mi djuno lo
du'u la .tom. du la .tomas.} and {mi djuno lo du'u la .tom. du la
.tom.}?

Supposedly a du'u is a predication, a bridi. The following both seem true to me:
{lu la .tom. du la .tomas. li'u bridi fi la .tom. ce'o la .tomas.}
{la .tom. ce'o la .tomas. du la .tom. ce'o la .tom.}

Therefore, I'd think {lu la .tom. du la .tomas. li'u bridi fi la .tom.
ce'o la .tom.} would be true as well, which makes me think that {mi
djuno lo du'u la .tom. du la .tomas.} and {mi djuno lo du'u la .tom.
du la .tom.} are equivalent. Is this right?

I think I must have misunderstood something along the way, because if
{lu co'e li'u bridi} is true, and a du'u really is just a bridi, I'd
think we could just use lu/li'u in place of du'u. But, despite my
misunderstandings (and whatever stupid mistakes I surely made), I hope
you can understand the real point of my question.