Add "missing constructor" precondition violation example

unison-src/transcripts/merge2.md

@@ -450,3 +450,28 @@ proj/main> merge2 /topic
 ```ucm:hide
 .> project.delete proj
 ```
+
+### Missing constructor
+
+```ucm:hide
+.> project.create-empty proj
+proj/main> builtins.mergeio
+```
+
+```unison
+unique type Foo = Bar | Baz
+```
+
+```ucm
+proj/main> add
+proj/main> branch topic
+proj/topic> delete.term Foo.Bar
+```
+
+```ucm:error
+proj/main> merge2 /topic
+```
+
+```ucm:hide
+.> project.delete proj
+```

unison-src/transcripts/merge2.output.md

@@ -887,7 +887,6 @@ proj/main> merge2 /topic
 ```
 ### Constructor alias
 
-
 ```unison
 unique type Foo = Bar
 ```
@@ -931,3 +930,47 @@ proj/main> merge2 /topic
   declaration must have exactly one name for each constructor.
 
 ```
+### Missing constructor
+
+```unison
+unique type Foo = Bar | Baz
+```
+
+```ucm
+
+  Loading changes detected in scratch.u.
+
+  I found and typechecked these definitions in scratch.u. If you
+  do an `add` or `update`, here's how your codebase would
+  change:
+  
+    ⍟ These new definitions are ok to `add`:
+    
+      type Foo
+
+```
+```ucm
+proj/main> add
+
+  ⍟ I've added these definitions:
+  
+    type Foo
+
+proj/main> branch topic
+
+  Done. I've created the topic branch based off of main.
+  
+  Tip: Use `merge /topic /main` to merge your work back into the
+       main branch.
+
+proj/topic> delete.term Foo.Bar
+
+  Done.
+
+```
+```ucm
+proj/main> merge2 /topic
+
+  Missing constructor name.
+
+```