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doc/manual/source/language/expression/composition.md

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+# Composition Expression
+
+A composition expression combines multiple values, constraints, or structures.
+
+## `&`
+
+`&` performs constraint-preserving composition.
+
+```dcdl
+Port = Int & >= 1 & <= 65535;
+Config = MyConfig & { port = 8000; };
+```
+
+Because object fields present on only one side are retained, `&` has no direction.
+Use [`as`](./ascription.md) to verify that the left side is narrower than the right and treat the right side as the field domain.
+
+## `//`
+
+`//` performs a right-biased structural patch.
+
+```dcdl
+Patched = Base // {
+    feature_hoge.enable = false;
+};
+```
+
+See [Operators](../operators.md) for details.