Address review suggestions

Author: odersky

docs/_docs/reference/experimental/capture-checking/mutability.md

@@ -62,7 +62,7 @@ can only access exclusive capabilities defined in the class or passed to it in p
 For instance, we can also define counter objects that update a shared variable that is external to the object:
 
 ```scala
-object Registry extends Mutable:
+object Registry extends Stateful:
   var sharedCount = 0
   update class CounterX:
     update def next: Int =
@@ -87,7 +87,7 @@ an update method or an update class as non-private member or constructor.
 
  1. The class does not retain any exclusive capabilities from its environment.
  2. The constructor does not take arguments that retain exclusive capabilities.
- 3. The class does not does not have fields that retain exclusive universal capabilities.
+ 3. The class does not have fields that retain exclusive universal capabilities.
 
 **Restriction:** If a class or trait extends `Stateful` all its parent classes or traits must either extend `Stateful` or be read-only.
 
@@ -138,7 +138,7 @@ But it also has two other properties that are explained in the following.
 
 ## Separate Classes
 
-Each time one creates a mutable type one gets a separate fresh object that can be updated independently
+Each time one creates a value of a mutable type one gets a separate fresh object that can be updated independently
 of other objects. This property is expressed by extending the `Separate` trait in the `scala.caps` object:
 ```scala
 trait Separate extends ExclusiveCapability

tests/neg-custom-args/captures/mut-outside-mutable.check

@@ -6,3 +6,7 @@
 9 |    update def baz() = 1 // error
   |               ^
   |               Update method baz must be declared in a class extending the `Mutable` trait.
+-- Error: tests/neg-custom-args/captures/mut-outside-mutable.scala:14:15 -----------------------------------------------
+14 |    update def baz() = 1 // error
+   |               ^
+   |               Update method baz must be declared in a class extending the `Mutable` trait.

tests/neg-custom-args/captures/mut-outside-mutable.scala

@@ -8,3 +8,7 @@ trait Foo extends Mutable:
   def bar =
     update def baz() = 1 // error
     baz()
+
+trait Bar extends Mutable:
+  class Baz:
+    update def baz() = 1 // error

tests/neg-custom-args/captures/ref-with-file.check

@@ -0,0 +1,31 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/ref-with-file.scala:15:12 --------------------------------
+15 |  withFile: f => // error
+   |            ^
+   |Found:    (f: File^'s1) ->'s2 Ref[File^'s3]^
+   |Required: (f: File^²) => Ref[File^'s4]^'s5
+   |
+   |Note that capability cap cannot be included in capture set {} of value r.
+   |
+   |where:    =>  refers to a fresh root capability created in method Test when checking argument to parameter op of method withFile
+   |          ^   refers to a fresh root capability classified as Unscoped in the type of value r
+   |          ^²  refers to the universal root capability
+   |          cap is a fresh root capability created in anonymous function of type (f: File^'s1): Ref[File^'s3]^ of parameter parameter f² of method $anonfun
+16 |    val r = Ref(f)
+17 |    r
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/ref-with-file.scala:18:12 --------------------------------
+18 |  withFile: f => // error
+   |            ^
+   |Found:    (f: File^'s6) ->'s7 (??? : -> Nothing)
+   |Required: (f: File^) => Nothing
+   |
+   |Note that capability cap cannot be included in capture set {} of value r.
+   |
+   |where:    =>  refers to a fresh root capability created in method Test when checking argument to parameter op of method withFile
+   |          ^   refers to the universal root capability
+   |          cap is a fresh root capability created in anonymous function of type (f: File^'s6): (??? : -> Nothing) of parameter parameter f² of method $anonfun
+19 |    val r = Ref(f)
+20 |    ???
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/ref-with-file.scala

@@ -0,0 +1,23 @@
+import caps.*
+
+class Ref[T](init: T) extends Mutable, Unscoped:
+  var x = init
+  def get: T = x
+  update def put(y: T): Unit = x = y
+
+class File:
+  def read(): String = ???
+
+def withFile[T](op: (f: File^) => T): T =
+  op(new File)
+
+def Test =
+  withFile: f => // error
+    val r = Ref(f)
+    r
+  withFile: f => // error
+    val r = Ref(f)
+    ???
+
+
+