53 21.5 The Combo Language 55 21.6 Normal Forms Postulated to Provide Tractable Representations 55 21.6.1 A Simple Type System 56 21.6.2 Boolean Normal Form 57 21.6.3 Number Normal Form 57 21.6.4 List Normal Form 57 21.6.5 Tuple Normal Form 57 21.6.6 Enum Normal Form 58 21.6.7 Function