PEP 416

* Rephrase the first Rationale paragraph
 * Add 2 more use cases: cache and sandbox
 * Fix a typo

pep-0416.txt

@@ -19,16 +19,20 @@ Add a new frozendict builtin type.
 Rationale
 =========
 
-A frozendict mapping cannot be changed, but its values can be mutable (not
-hashable). A frozendict is hashable and so immutable if all values are hashable
-(immutable).
+A frozendict is a read-only mapping: a key cannot be added nor removed, and a
+key is always mapped to the same value. However, frozendict values can be
+mutable (not hashable). A frozendict is hashable and so immutable if and only
+if all values are hashable (immutable).
 
 Use cases of frozendict:
 
+ * frozendict can be used to implement a cache
  * hashable frozendict can be used as a key of a mapping or as a member of set
  * frozendict helps optimization because the mapping is constant
  * frozendict avoids the need of a lock when the frozendict is shared
    by multiple threads or processes, especially hashable frozendict
+ * frozendict helps to implement a security sandbox with read-only objects,
+   e.g. freeze __builtins__ mapping
 
 
 Constraints
@@ -47,7 +51,7 @@ Implementation
    "Py_hash_t hash;" field
  * frozendict.__hash__() is implemented using hash(frozenset(self.items())) and
    caches the result in its private hash attribute
- * Register frozendict has a collections.abc.Mapping
+ * Register frozendict as a collections.abc.Mapping
  * frozendict can be used with PyDict_GetItem(), but PyDict_SetItem() and
    PyDict_DelItem() raise a TypeError