:   Description of the type used in documentation. Give information of
    the type and any of its arguments.

`check`

:   A function to type check the definition value. Takes the definition
    value as a parameter and returns a boolean indicating the type check
    result, `true` for success and `false` for failure.

`merge`

:   A function to merge multiple definitions values. Takes two
    parameters:

    *`loc`*

    :   The option path as a list of strings, e.g. `["boot" "loader
                 "grub" "enable"]`.

    *`defs`*

    :   The list of sets of defined `value` and `file` where the value
        was defined, e.g. `[ {
                 file = "/foo.nix"; value = 1; } { file = "/bar.nix"; value = 2 }
                 ]`. The `merge` function should return the merged value
        or throw an error in case the values are impossible or not meant
        to be merged.

`getSubOptions`

:   For composed types that can take a submodule as type parameter, this
    function generate sub-options documentation. It takes the current
    option prefix as a list and return the set of sub-options. Usually
    defined in a recursive manner by adding a term to the prefix, e.g.
    `prefix:
          elemType.getSubOptions (prefix ++
          ["prefix"])` where *`"prefix"`* is the newly added prefix.

`getSubModules`

:   For composed types that can take a submodule as type parameter, this
    function should return the type parameters submodules. If the type
    parameter is called `elemType`, the function should just recursively
    look into submodules by returning `elemType.getSubModules;`.

`substSubModules`

:   For composed types that can take a submodule as type parameter, this
    function can be used to substitute the parameter of a submodule
    type. It takes a module as parameter and return the type with the
    submodule options substituted. It is usually defined as a type
    function call with a recursive call to `substSubModules`, e.g for a
    type `composedType` that take an `elemtype` type parameter, this
    function should be defined as `m:
          composedType (elemType.substSubModules m)`.

`typeMerge`

:   A function to merge multiple type declarations. Takes the type to
    merge `functor` as parameter. A `null` return value means that type
    cannot be merged.

    *`f`*

    :   The type to merge `functor`.

    Note: There is a generic `defaultTypeMerge` that work with most of
    value and composed types.

`functor`

:   An attribute set representing the type. It is used for type
    operations and has the following keys:

    `type`

    :   The type function.

    `wrapped`

    :   Holds the type parameter for composed types.

    `payload`

    :   Holds the value parameter for value types. The types that have a
        `payload` are the `enum`, `separatedString` and `submodule`
        types.

    `binOp`

    :   A binary operation that can merge the payloads of two same
        types. Defined as a function that take two payloads as
        parameters and return the payloads merged.