(CLL:) "The abstractor si'o expresses a mental image, a concept, an idea:

9.2) mi nelci le si'o la lojban. cu mulno
I enjoy the concept-of Lojban being-complete."

I have mostly considered si'o broda a shorthand for sidbo be le broda bei zu'i:

sidbo x1 (idea abstract) is an idea/concept/thought about x2 (object/abstract) by thinker x3 (also (adjective:) x1 is ideal/ideational); (cf. ciksi, jijnu, mucti, jinvi, nabmi, pensi, xanri, cmavo list si'o)

My usage precedes the invention of ce'u & perhaps has not evolved accordingly, but see:

"7. In si'o abstractions, all elided sumti are interpreted as ce'u" as here.

Your use of si'o was a prime consideration in coming up with (7), what with you appearing to be one of the main users of si'o. --And