PEP 827: Minor tweaks to the examples

 * Add Exclude and Extract to the TS-style utility types
 * Add an `age` field to User in the prisma style example so that we
   aren't selecting *all* non-id fields
 * Add some discussion about Property/Link/MultiLink, since there
   was some confusion where they came from

Author: msullivan

peps/pep-0827.rst

@@ -133,9 +133,16 @@ the database) like::
         id: Property[int]
 
         name: Property[str]
+        age: Property[int | None]
         email: Property[str]
         posts: Link[Post]
 
+
+(In the example, ``Property`` indicates a scalar type, ``Link``
+indicates a reference to another table, and ``MultiLink`` indicates a
+potentially many-to-many reference to another table; all would be
+defined by the ORM library.)
+
 So, in Python code, a call like::
 
     db.select(
@@ -1216,6 +1223,7 @@ We present implementations of a selection of them::
 
     # Omit<T, Keys>
     # Constructs a type by picking all properties from T and then removing Keys.
+    # Note that unlike in TS, our Omit does not depend on Exclude.
     type Omit[T, Keys] = typing.NewProtocol[
         *[
             p
@@ -1224,6 +1232,26 @@ We present implementations of a selection of them::
         ]
     ]
 
+    # Exclude<T, U>
+    # Constructs a type by excluding from T all union members assignable to U.
+    type Exclude[T, U] = Union[
+        *[
+            x
+            for x in typing.Iter[typing.FromUnion[T]]
+            if not typing.IsAssignable[x, U]
+        ]
+    ]
+
+    # Extract<T, U>
+    # Constructs a type by extracting from T all union members assignable to U.
+    type Extract[T, U] = Union[
+        *[
+            x
+            for x in typing.Iter[typing.FromUnion[T]]
+            if typing.IsAssignable[x, U]
+        ]
+    ]
+
     # Partial<T>
     # Constructs a type with all properties of T set to optional (T | None).
     type Partial[T] = typing.NewProtocol[