Add some tests

unison-src/transcripts/fix2350.md

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+
+This tests an issue where ability variables were being defaulted over
+eagerly. In general, we want to avoid collecting up variables from the
+use of definitions with types like:
+
+    T ->{e} U
+
+Since this type works for every `e`, it is, 'pure;' and we might as
+well have `e = {}`, since `{}` is a subrow of every other row.
+However, if `e` isn't just a quantified variable, but one involved in
+ongoing inference, it's undesirable to default it. Previously there
+was a check to see if `e` occurred in the context. However, the wanted
+abilities being collected aren't in the context, so types like:
+
+    T ->{S e} U ->{e} V
+
+were a corner case. We would add `S e` to the wanted abilities, then
+not realize that `e` shouldn't be defaulted.
+
+```unison
+unique ability Storage d g where
+  save.impl : a ->{Storage d g} ('{g} (d a))
+
+save : a ->{Storage d g, g} (d a)
+save a = !(save.impl a)
+```

unison-src/transcripts/fix2350.output.md

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+
+This tests an issue where ability variables were being defaulted over
+eagerly. In general, we want to avoid collecting up variables from the
+use of definitions with types like:
+
+    T ->{e} U
+
+Since this type works for every `e`, it is, 'pure;' and we might as
+well have `e = {}`, since `{}` is a subrow of every other row.
+However, if `e` isn't just a quantified variable, but one involved in
+ongoing inference, it's undesirable to default it. Previously there
+was a check to see if `e` occurred in the context. However, the wanted
+abilities being collected aren't in the context, so types like:
+
+    T ->{S e} U ->{e} V
+
+were a corner case. We would add `S e` to the wanted abilities, then
+not realize that `e` shouldn't be defaulted.
+
+```unison
+unique ability Storage d g where
+  save.impl : a ->{Storage d g} ('{g} (d a))
+
+save : a ->{Storage d g, g} (d a)
+save a = !(save.impl a)
+```
+
+```ucm
+
+  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`:
+    
+      unique ability Storage d g
+      save : a ->{g, Storage d g} d a
+
+```

unison-src/transcripts/fix2474.md

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+
+Tests an issue with a lack of generality of handlers.
+
+In general, a set of cases:
+
+  { e ... -> k }
+
+should be typed in the following way:
+
+  1. The scrutinee has type `Request {E, g} r -> s` where `E` is all
+     the abilities being handled. `g` is a slack variable, because all
+     abilities that are used in the handled expression pass through
+     the handler. Previously this was being inferred as merely
+     `Request {E} r -> s`
+  2. The continuation variable `k` should have type `o ->{E, g} r`,
+     matching the above types (`o` is the result type of `e`).
+     Previously this was being checked as `o ->{E0} r`, where `E0` is
+     the ability that contains `e`.
+
+```ucm
+.> builtins.mergeio
+```
+
+```unison
+structural ability Stream a where 
+  emit : a -> ()
+
+Stream.uncons : '{Stream a, g} r ->{g} Either r (a, '{Stream a, g} r)
+Stream.uncons s = 
+  go : Request {Stream a,g} r -> Either r (a, '{Stream a,g} r)
+  go = cases 
+    { r } -> Left r
+    { Stream.emit a -> tl } -> Right (a, tl : '{Stream a,g} r)
+  handle !s with go
+```

unison-src/transcripts/fix2474.output.md

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+
+Tests an issue with a lack of generality of handlers.
+
+In general, a set of cases:
+
+  { e ... -> k }
+
+should be typed in the following way:
+
+  1. The scrutinee has type `Request {E, g} r -> s` where `E` is all
+     the abilities being handled. `g` is a slack variable, because all
+     abilities that are used in the handled expression pass through
+     the handler. Previously this was being inferred as merely
+     `Request {E} r -> s`
+  2. The continuation variable `k` should have type `o ->{E, g} r`,
+     matching the above types (`o` is the result type of `e`).
+     Previously this was being checked as `o ->{E0} r`, where `E0` is
+     the ability that contains `e`.
+
+```ucm
+.> builtins.mergeio
+
+  Done.
+
+```
+```unison
+structural ability Stream a where 
+  emit : a -> ()
+
+Stream.uncons : '{Stream a, g} r ->{g} Either r (a, '{Stream a, g} r)
+Stream.uncons s = 
+  go : Request {Stream a,g} r -> Either r (a, '{Stream a,g} r)
+  go = cases 
+    { r } -> Left r
+    { Stream.emit a -> tl } -> Right (a, tl : '{Stream a,g} r)
+  handle !s with go
+```
+
+```ucm
+
+  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`:
+    
+      structural ability Stream a
+      Stream.uncons : '{g, Stream a} r
+                      ->{g} Either r (a, '{g, Stream a} r)
+
+```