[v3] Implement new Concept Exercise: `multi-arity functions`

Read docs/concept-exercises.md, docs/rationale-for-v3.md, docs/features-of-v3.md, and watch the linked anatomy video first. Then create the listed files under languages/clojure/exercises/concept/, using the learning objectives and out-of-scope topics as the content boundaries; done means the complete exercise exists without representer or analyzer changes.

This issue describes how to implement the multi-arity functions concept exercise for the Clojure track.

Getting started

Please please please read the docs before starting. Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before diving into the implementation, please read up on the following documents:

• The features of v3.
• Rationale for v3.
• What are concept exercise and how they are structured?

Please also watch the following video:

• The Anatomy of a Concept Exercise.

Goal

The goal of this exercise is to teach the student how multi-arity functions are implemented in Clojure.

Learning objectives

After completing this exercise the student will:

• Know how to define functions taking different numbers of parameters (arities)
• How to invoke them
• Creating a default value by having one arity call another

Out of scope

• Variadic functions
• Polymorphism

Concepts

• functions
• default-arguments
• evaluation
• multiple-dispatch
• return-values

Prerequisites

• basic-functions
• repl
• lists

Resources to refer to

Hints

• Clojure guide - functions

After

• Clojure reference - evaluation

Representer

This exercise does not require any modifications to the track's representer.

Analyzer

This exercise does not require any modifications to the analyzer.

Implementing

To implement a concept exercise, the following files must be created:

languages
└── clojure
    └── exercises
        └── concept
            └── <SLUG>
                ├── .docs
                |   ├── instructions.md
                |   ├── introduction.md
                |   ├── hints.md
                |   └── after.md (optional)
                ├── .meta
                |   |── design.md
                |   └── Example.clj
                ├── <NAME>.clj
                └── <NAME>_test.clj

Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.