///|
/// MoonCron - A cron expression parser and schedule calculator written in MoonBit.
///
/// MoonCron is a pure MoonBit implementation of a standard 5-field cron expression
/// parser, validator, matcher, and execution time calculator. It does NOT perform
/// actual task scheduling.
///
/// # Features
///
/// - Parse standard 5-field cron expressions
/// - Validate expressions with clear error messages
/// - Match DateTime values against cron expressions
/// - Compute next execution time(s)
/// - Generate human-readable descriptions (English and Chinese)
/// - Support for common aliases (@hourly, @daily, etc.)
///
/// # Quick Start
///
/// ```
/// let cron = CronExpr::parse("*/15 9-17 * * 1-5")
/// match cron {
///   Ok(expr) => {
///     println(expr.describe_en())
///     let times = expr.next_n(DateTime::new(2026, 7, 25, 12, 0), 5)
///     for time in times {
///       println(time.to_string())
///     }
///   }
///   Err(err) => println(cron_error_to_string(err))
/// }
/// ```