doc/tutorial/basics: add basics tutorial
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+[TOC](Readme.md) [Prev](conditional.md) _Next_
+
+# Null Coalescing
+
+With null coalescing we really mean error, or bottom, coalescing.
+The defaults mechanism for disjunctions can also be
+used to provide fallback values in case an expression evaluates to bottom.
+
+In the example the fallback values are specified
+for `a` and `b` in case the list index is out of bounds.
+
+<!-- CUE editor -->
+```
+list: [ "Cat", "Mouse", "Dog" ]
+
+a: list[0] | "None"
+b: list[5] | "None"
+```
+
+<!-- result -->
+```
+list: [ "Cat", "Mouse", "Dog" ]
+
+a: "Cat"
+b: "None"
+```
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