///
    /// # Example
    ///
    /// ```
    /// use anyhow::Error;
    /// use std::io;
    ///
    /// pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> {
    ///     for cause in error.chain() {
    ///         if let Some(io_error) = cause.downcast_ref::<io::Error>() {
    ///             return Some(io_error.kind());
    ///         }
    ///     }
    ///     None
    /// }
    /// ```