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docs/mainpage.md

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+# Introduction {#mainpage}
+
+This is the Doxygen API documentation for CLI11 parser. There is a friendly
+introduction to CLI11 on the [GitHub page](https://github.com/CLIUtils/CLI11),
+and [a tutorial series](https://cliutils.github.io/CLI11/book/).
+
+The main classes are:
+
+| Name           | Where used                                                |
+| -------------- | --------------------------------------------------------- |
+| CLI::Option    | Options, stored in the app                                |
+| CLI::App       | The main application or subcommands                       |
+| CLI::Validator | A check that can affect the type name                     |
+| CLI::Formatter | A subclassable formatter for help printing                |
+| CLI::ExitCode  | A scoped enum with exit codes                             |
+| CLI::Timer     | A timer class, only in CLI/Timer.hpp (not in `CLI11.hpp`) |
+| CLI::AutoTimer | A timer that prints on deletion                           |
+
+Groups of related topics:
+
+| Name                 | Description                                    |
+| -------------------- | ---------------------------------------------- |
+| @ref error_group     | Errors that can be thrown                      |
+| @ref validator_group | Common validators used in CLI::Option::check() |