pub fn process_program(program: &Program) -> WeldResult<Expr> {
    let mut all_macros = STANDARD_MACROS.with(|v| v.clone());
    all_macros.extend(program.macros.iter().cloned());
    let mut expr = process_macros(&program.body, &all_macros)?;
    process_type_aliases(&mut expr, program.type_aliases.clone())?;
    Ok(expr)
}
Apply macros to a program, including the standard macros built into Weld.
weld/src/syntax/macro_processor.rs