Function library_function_rule 

pub fn library_function_rule(
    kind: ExportKind,
    name: &str,
    key: &str,
    lib: Expr,
    native_sig: Option<(Vec<Expr>, Expr)>,
) -> RuleEntry

One key -> Caller[LibraryFunctionLoad[...]] association rule. key is the final association key (see export_key); name is the exported C symbol.