Turn exercise 009 into practice

[cuihtlauac]

Extract the OCaml code from the markdown file corresponding to this exercise. The file is: data/exercise/009_NAME.md (where 009 is the number from the title of this issue and NAME is the rest of the file name, for instance data/exercise/001_tail.md is the file which corresponds to exercise 001). Use the code in this file to create a folder practice/009 (using this same number).

Populate the 009 folder with (almost) the same files as in folders practice/001 and practice/002 (they are examples).

Three files are different in each folder:

1. practice/009/ex.ml: it contains the function's signature, which needs to be implemented by the learner;
2. practice/009/work/impl.ml: it includes the template meant to be filed by the learner;
3. practice/009/answer/impl.ml: consists of the solution from the original markdown file.

The examples included in the original markdown file must be turned into test cases in the practice/009/ex.ml file.

Folder structure to create:

practice/009/
├── answer
│   ├── impl.ml
│   └── test
│       ├── dune
│       └── run.ml
├── dune
├── dune-project
├── ex.ml
└── work
    ├── impl.ml
    └── test
        ├── dune
    

[Kxrishx03]

Hii! @cuihtlauac Can I take part in it?

[cuihtlauac]

Yes @Kxrishx03, go for it

[jahielkomu]

Hi @Kxrishx03, I hope you are well.
Are you working on this?

[Kxrishx03]

@jahielkomu I've completed the task, waiting for PR to get approved.

[cuihtlauac]

@jahielkomu I've completed the task, waiting for PR to get approved.

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