统一token type
血淋淋的重构
command line model
can define a identifier
define SymbolTable
implement symbol table
adjust the relation between env and symbol table
output help
define function 1st version
interpreter implements
nothing special
boolean operator reloading
方法定义初次运作
LL1_DEF()函数定义完毕后应该关闭计算模型，这样可以防止在对未赋值参数进行计算
generate a tree for cdr and car functions
correct the wrong concept that actual argument can be a procedure, cons, or list, not only a number
implement a deepcopy function for AST pointer
solve the argument duplicate problem
the domain of the function will be solved later
new folder