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diff --git a/dhall/src/errors/HandlerNotAFunction.txt b/dhall/src/errors/HandlerNotAFunction.txt
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+Explanation: You can ❰merge❱ the alternatives of a union using a record with one
+handler per alternative, like this:
+
+
+    ┌─────────────────────────────────────────────────────────────────────┐
+    │     let union    = < Left = +2 | Right : Bool >                     │
+    │ in  let handlers = { Left = Natural/even, Right = λ(x : Bool) → x } │
+    │ in  merge handlers union : Bool                                     │
+    └─────────────────────────────────────────────────────────────────────┘
+
+
+... as long as each handler is a function
+
+For example, the following expression is $_NOT valid:
+
+
+    ┌─────────────────────────────────────────┐
+    │ merge { Foo = True } < Foo = 1 > : Bool │
+    └─────────────────────────────────────────┘
+                    ⇧
+                    Invalid: Not a function
+
+
+Your handler for this alternative:
+
+↳ $txt0
+
+... has the following type:
+
+↳ $txt1
+
+... which is not the type of a function