R语言是如何将变量值和变量绑定的
在r语言中，当前的 workspace就是global enviroment，当输入变量名时，首先会在global enviroment中搜索该变量，如有，则将它显示出来。
第二步，如在global enviroment中没有找到该变量民，则搜索search list中的各个包，search list 中的内容可以用search()得到
如果用户使用library()load了一个package ，则这个package将在search list中处于第二的位置
Lexical scoping in R means that:
the values of free variables are searched for in the environment in which the function
was defined.
If the value of a symbol is not found in the environment in which a function was defined,
then the search is continued in the parent environment.
• The search continues down the sequence of parent environments until we hit the top-level
environment; this usually the global environment (workspace) or the namespace of a package.
• After the top-level environment, the search continues down the search list until we hit the
empty environment.
If a value for a given symbol cannot be found once the empty environment is arrived at, then an
error is thrown.