/**
    Create a list consisting of a single element.  `singleton x` is
    sometimes more convenient with respect to indentation than `[x]`
    when x spans multiple lines.

    # Example

    ```nix
    singleton "foo"
    => [ "foo" ]
    ```

    # Type

    ```
    singleton :: a -> [a]
    ```

    # Arguments

    - [x]

  */