  The fundamental theorem of symmetric polynomials over rings is a classical
result which states that every unital commutative ring is fully elementary,
i.e. we can express symmetric polynomials with elementary ones in a unique way.
The result does not extend directly to polynomials over semirings, but we do
have analogous results for some special semirings, for example, the tropical,
extended and supertropical semirings. These all fall into a larger class of
upper-bound semirings. In this paper we extend the known results and give a
complete characterization of fully elementary upper-bound semirings. We further
improve this characterization statement in the case of linearly ordered
upper-bound semirings.
