Macro code_enum 

macro_rules! code_enum {
    (
        $(#[$meta:meta])*
        $name:ident {
            $($(#[$vmeta:meta])* $code:literal => $variant:ident),* $(,)?
        }
    ) => { ... };
}

Define an X12 code-list data element as an enum.

Generates the enum (one variant per code plus an Unknown(String) catch-all so unknown/future codes still round-trip), its Display (variant -> code string), X12Element parsing (code string -> variant), and serde (as the code string, so JSON stays a plain string). The variant identifiers are supplied explicitly because many X12 codes are not valid Rust identifiers (e.g. “9L”, “C1”).

At full scale (element 737 has 196 codes, 738 ~600, 355 has 844) these invocations are expected to be machine-generated from the code lists.