Add top and bottom type operators (wip).

See issue #94.

Still need to:

-   Add them to taxonomy.
-   Handle top and bottom types in transformation graphs.
-   Add tests.
-   Replace wildcards in query types with `Bottom`.

transformation_algebra/type.py

@@ -217,8 +217,10 @@ def __call__(self, *params: Type) -> TypeOperation:
 
     def subtype(self, other: TypeOperator, strict: bool = False) -> bool:
         assert isinstance(other, TypeOperator)
-        return ((not strict and self == other) or
-            bool(self.parent and self.parent.subtype(other)))
+        return (not strict and self is other) or (
+            self is Bottom or other is Top or
+            bool(self.parent and self.parent.subtype(other))
+        )
 
     def normalize(self) -> TypeInstance:
         return self.instance()
@@ -444,7 +446,7 @@ def match(self, other: TypeInstance, subtype: bool = False,
                 return a._operator == b._operator or \
                     (subtype and a._operator.subtype(b._operator))
             elif a._operator != b._operator:
-                return False
+                return a._operator is Bottom or b._operator is Top
             else:
                 result: Optional[bool] = True
                 for v, s, t in zip(a._operator.variance, a.params, b.params):
@@ -948,6 +950,12 @@ def fulfill(self) -> bool:
 "The special constructor for the unit type."
 Unit = TypeOperator('Unit')
 
+"The bottom type contains no values and is a subtype to everything."
+Bottom = TypeOperator('Bottom')
+
+"The top type contains all values and is a supertype to everything."
+Top = TypeOperator('Top')
+
 "A wildcard: fresh variable, unrelated to, and matchable with, anything else."
 _ = TypeSchema(lambda: TypeVariable(wildcard=True))