1. {bu} is used after vowels to anaphorically reference LA/LE-sumti with sumti tails that start with the same vowel.
2. {bu} is proposed by gejyspa in {nu bu} and {du'u bu} to reference sumti that have sumti tails that start (or probably contain at main level) {nu} or {du'u} abstractions.
This immediately raises a question: can a {nu bu} according to rule 1. reference {lo nupre}? If not 2. is indeed a new rule.
3. {bu} is proposed to be used in mekso to reference certain symbols like {.alfas. bu} for alpha. Probably it's about {me'o .alfas. bu} since we are talking about formulae. In this case it doesn't conflict with 1. and 2. and just adds this new third rule, namely, that inside {me'o} strings such constructs as {abu}, {nu bu}, {.alfas. bu} get a new meaning strictly defined in the dictionary.
4. Using {denpa bu} for "dot" without {me'o} can't fit into rules 1. and 2. From rule 1. and 2. we can assume that it anaphorically references {LE/LA denpa}. Neither using {.alfas. bu} without {me'o} can reference the symbol alpha. Instead, it would reference {la .alfas.}.
5. Neither {uibu} can be used without {me'o} to denote smileys.
If {denpa bu} and {.alfas. bu} can be used without {me'o} to reference dots and alpha symbols then a,e,i,o,u, nu,du'u are exclusions to this rule. And in this case we aren't able to reference "a" as a symbol without {me'o} since {abu} is always an anaphorical marker.
So my proposal is:
1. to always use {me'o} to reference symbols or to use fu'ivla for them like {denpabu} and {slakabu}.
2. The letteral meanings of uibu, denpa bu, slaka bu are to be defined in the dictionary.
3. {denpa bu} means {lo denpa} referenced anaphorically, it is close to {le} explained
here or
ri'oi. But even more correctly would be to show two examples identical in meaning:
a) {lo denpa cu pendo mi .ije dy. xendo}
b) {lo denpa cu pendo mi .ije denpa bu xendo}
Clearly, {denpa bu prenu bu} would reference {lo denpa prenu} (no matter how often one would need that).
{mi bu} would refer to {lo mi broda}.
{ca bu} would refer to {lo ca broda} or {i ca broda} and thus be similar to {lo
go'oi}.
Interesting that in past people suggested using different {SOMETHING bu} to mark keyboard keys. No wonder {a bu} was never suggested as a name of A keyboard key because it's already taken by an anaphorical construct.