Macro hopper_validate 

macro_rules! hopper_validate {
    (
        accounts = $accounts:expr,
        program_id = $program_id:expr,
        data = $data:expr,
        rules {
            $( $rule:expr ),+ $(,)?
        }
    ) => { ... };
}

Build a validation pipeline declaratively.

Each rule is a combinator that returns impl Fn(&ValidationContext) -> ProgramResult. The macro creates a context, enforces unique writable accounts by default, and then invokes each rule in order (fail-fast).