doc/tutorial/basics: add basics tutorial
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+[TOC](Readme.md) [Prev](unification.md) [Next](disjstruct.md)
+
+# Disjunctions
+
+Disjunctions, or sum types, define a new type that is one of several things.
+
+In the example, `conn` defines a `protocol` field that must be one of two
+values: `"tcp"` or `"upd"`.
+It is an error for a concrete `conn`
+to define anything else than these two values.
+
+<!-- CUE editor -->
+```
+conn: {
+ address: string
+ port: int
+ protocol: "tcp" | "udp"
+}
+
+lossy: {
+ address: "1.2.3.4"
+ port: 8888
+ protocol: "udp"
+}
+```
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