Combine
CompoundExpression(Combination)
Abstract
Naming
expression：可以直接求值，得到一个值。procedure：需要提供操作数，才可以求值，得到一个值。
Variable(Name)
+ - * /
Data
CompoundProcedure
procedure definition/block structure/lambda
Evaluating(general/specialized rules)
Procedure
Number
Apply: Substitution Model
Primitive
Nesting
