* [Composing custom error types](#composing-custom-error-types)
* [Advice for library writers](#advice-for-library-writers)
* [Case study: A program to read population data](#case-study-a-program-to-read-population-data)
* [Initial setup](#initial-setup)
* [Argument parsing](#argument-parsing)
* [Writing the logic](#writing-the-logic)
* [Composing custom error types](#composing-custom-error-types)
* [Advice for library writers](#advice-for-library-writers)
* [Case study: A program to read population data](#case-study-a-program-to-read-population-data)
* [Initial setup](#initial-setup)
* [Argument parsing](#argument-parsing)
* [Writing the logic](#writing-the-logic)