I fully agree. the ":" prefix or suffix also works if you dont like the "!"
I tend to keep "@" reserved only for use for annotations (that are additional optional markup embedded in the source code, but that can be compiled and run without taking them into account; annotations define a separate language with its own API, like in Java). Using annotations in a Lua-based API is something to be specified later, but it should already have a reserved syntax allowing them to be longer than a single keyword, with some parameter list).
Annotation are very productful for code generation (including automated replacement of code fragments), code analysis (when the Lua parser alone is not sufficient and what needs to be documented is a runtime behavior or some usage contraints for libraries), automated code documentation (comments in Lua are not enough, as they are in free form. However "@" is already used to embed annotations within Lua comments for the same purpose, but it's hard to use annotations in a Lua compiler to give non-imperative *hints* to the compiler, if it needs to also parse the content of comments, that the normal Lua parser has already dropped, this requires a hook inside the Lua parser when it parses the content of comments before dropping them, so that it can feed some external metadata storage in the abstract sytnax tree that is built by the normal parser)
