I've realized I've left off a theory-- that "pu" is a possible clitic, maybe like a verbal comma or semi-colon.
If it is a semi-colon like English, then it would join related sentences that seem like they're missing the co-ordinating conjunction, such as For, Nor, And, But, Or, Yet, So, Like. (seems unlike, see below)
OR it joins related sentences where there is a conjuctive adverb, like furthermore, elsewhere, equally, hence, henceforth, however, just as, etc. (seems unlike, see below)
OR it splits lists of lists, e.g. apple, oranges; booze, beer, vodkas (unlikely, this would be mostly useless)
I'm not sure why it would be a co-ordinated conjuntion, because we already have "taso". And if we want to allow more, it would make more sense to overload, "en", "anu", "sama", "ala", "tawa" and allow them as sentence intials, the way "taso" is legal at the start of a sentence now.
If it is a semicolon like programming, then "pu" is a verbal period (end of sentence marker!) Unlikely, but potentially useful. Sentence splits can cause extra ambiguity.
If it is a semicolon like in Church Slavonic, then "pu" is a verbal question mark! Again, unlikely. If we get a end of sentence marker, it will cover all types of sentence endings (!, ., ..., and ?)
If it is a comma, as in in a separation of lists, that is unlikely, we have "li-chains" "e-chains" and prep-chains, already. So this is unlikely.
If it is a comma, as in the puctionation that joins the dependent and indepedent and dependent clauses, i.e. pu would be like the word "that".
Personally, I think it would be awesome to get relative clauses.
Here is why:
mi lukin e soweli pu moku e kasi pi ma supa. I saw the cow that eats the grass of the plains. I saw the grass-eating plains animal. An action is the salient feature of this animal.
This also might be usable when the salient feature is a prep-phrase (i.e. the prep phrase modifies something other than the verb)
mi lukin e soweli ni: soweli li moku e kasi pi ma supa. (using e-ni-clauses) I see this animal, the animal is eating the plants of the plains. This kind of missing the point, that the salient feature of the cow is that it eats grass on the plains.
mi lukin e soweli. soweli li moku e kasi pi ma supa. I see an animal. The animal is eating plans of the plains. Same problem as above and worse, it makes it sound like the topic has decidenly switched to the animals eating-- where as "pu" as relative clause marker lets the reader know that we are still talking about a cow, which may or may not be eating right now.
mi lukin e soweli pi kasi moku pi ma supa. I see the animal of the edible plants of the plains. "pi" is so semantically bleached, it doesn't signal that exactly what the relationship between the soweli and the kasi is (namely that one eats the other... a cow isn't a type of plant, it doesn't have a plant, etc)
UPDATE. This also seems to be leading towards the Japanese ! Exciting stuff for the easily excitable, like myself