Notate class hierarchy as semilattice

If we were to imagine 'is-subclass-of' order-theoretically,
then the class hierarchy is a ∨-semilattice.

MachineArithmetic-MathNotation/Class.extension.st

@@ -0,0 +1,28 @@
+Extension { #name : #Class }
+
+{ #category : #'*MachineArithmetic-MathNotation' }
+Class >> ∪ [ anotherClass
+	"Join in the semilattice of classes partially-ordered by 'is-subclass-of'.
+	
+	SmallInteger ∪ Association >>> Magnitude
+	"
+
+	"Join of any two comparable elements is the 'bigger' one"
+	self ⊆ anotherClass ifTrue: [ ^anotherClass ].
+	anotherClass ⊆ self ifTrue: [ ^self ].
+
+	^self superclass ∪ anotherClass
+]
+
+{ #category : #'*MachineArithmetic-MathNotation' }
+Class >> ⊆ [ anotherClass
+	"Answer whether the receiver is a subclass of anotherClass,
+	 reflexive case included.
+	
+	Integer ⊆ Object >>> true
+	Integer ⊆ String >>> false
+	"
+
+	self = anotherClass ifTrue: [ ^true ].
+	^self superclass ⊆ anotherClass
+]

MachineArithmetic-MathNotation/UndefinedObject.extension.st

@@ -0,0 +1,6 @@
+Extension { #name : #UndefinedObject }
+
+{ #category : #'*MachineArithmetic-MathNotation' }
+UndefinedObject >> ⊆ [ aClass 
+	^false
+]