///
/// # Example
///
/// ```
/// # type V = ();
/// #
/// use anyhow::{anyhow, Result};
///
/// fn lookup(key: &str) -> Result<V> {
///     if key.len() != 16 {
///         return Err(anyhow!("key length must be 16 characters, got {:?}", key));
///     }
///     
///     // ...
///     # Ok(())
/// }
/// ```