Clojure is a Lisp variant, so evaluation occurs by first reading an expression as a Clojure data structure (usually a list), then evaluating each expression in the data structure.
Clojure是一种Lisp变体，因此计算过程首先将一个表达式读取为Clojure数据结构(通常是一个列表)，随后计算数据结构中的各表达式。
Each expression conveys a set of related messages.
每个表情都传达着一系列相关信息。
Each expression class has an evaluate method, and an enumerate method.
每个表达式类都有一个evaluate方法和一个enumerate方法。
Each expression class has a simplify method, to allow expression rewriting.
每个表达式类都有一个simplify方法，以允许表达式重写。
Each expression returns a list of values that can be used as input to other expressions.
每个表达式返回一系列的值，这些值又可以作为其他表达式的输入。
Each expression returns a value that depends only on the operands of the expression, and no expression has any side effects.
每个表达式仅根据其操作数来返回值，并且都不会产生副作用。
Each expression returns a value that depends only on the operands of the expression, and no expression has any side effects.
每个表达式仅根据其操作数来返回值，并且都不会产生副作用。
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