doc/tutorial/basics: add basics tutorial
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+[TOC](Readme.md) [Prev](duplicates.md) [Next](types.md)
+
+# Bottom
+
+Specifying duplicate fields with conflicting values results in an error,
+denoted `_|_`.
+
+Technically speaking, bottom is just a value like any other.
+But for all practical purposes it is okay to think of the bottom value
+as an error.
+
+Note that an error is different from `null`: `null` is a valid JSON value,
+whereas `_|_` is not.
+
+<!-- CUE editor -->
+```
+a: 4
+a: 5
+
+l: [ 1, 2 ]
+l: [ 1, 3 ]
+```
+
+<!-- result -->
+```
+a: _|_
+l: _|_
+```
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