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diff --git a/src/content/chapter1_functions/lesson099_documentation_comments/code.gleam b/src/content/chapter1_functions/lesson099_documentation_comments/code.gleam
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+//// A module containing some unusual functions and types.
+
+/// A type where the value can never be constructed.
+/// Can you work out why?
+pub type Never {
+  Never(Never)
+}
+
+/// Call a function twice with an initial value.
+///
+pub fn twice(argument: value, function: fn(value) -> value) -> value {
+  function(function(argument))
+}
+
+/// Call a function three times with an initial value.
+///
+pub fn thrice(argument: value, function: fn(value) -> value) -> value {
+  function(function(function(argument)))
+}
diff --git a/src/content/chapter1_functions/lesson099_documentation_comments/text.html b/src/content/chapter1_functions/lesson099_documentation_comments/text.html
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+<p>
+  Documentation and comments are important tools for making your code easier to
+  work with and understand.
+</p>
+<p>
+  As well as regular <code>//</code> comments Gleam has <code>///</code> and
+  <code>////</code> comments which are used for attaching documentation to code.
+</p>
+<p>
+  <code>///</code> is used for documenting types and functions, and should be
+  placed immediately before the type or function it is documenting.
+</p>
+<p>
+  <code>////</code> is used for documenting modules, and should be placed
+  at the top of the module.
+</p>