diff --git a/version0.7/pda_demo.cpp b/version0.7/pda_demo.cpp
new file mode 100644
index 0000000..8693229
--- /dev/null
+++ b/version0.7/pda_demo.cpp
@@ -0,0 +1,256 @@
+#include <list>
+#include <stack>
+#include <cstring>
+#include <iostream>
+
+enum token_type
+{
+	e=1,E,_E,T,_T,F,I,__add,__sub,__mul,__div,__left_curve,__right_curve,sharp
+};
+
+const int max_len=3;
+struct cmp_seq
+{
+	int tokens[max_len];
+	int res;
+};
+cmp_seq par[]=
+{
+	{{T,_E},                        E},
+	{{__add,T,_E},                 _E},
+	{{__sub,T,_E},                 _E},
+	{{e},                          _E},
+	{{F,_T},                        T},
+	{{__mul,F,_T},                 _T},
+	{{__div,F,_T},                 _T},
+	{{e},                          _T},
+	{{__left_curve,E,__right_curve},F},
+	{{I},                           F}
+};
+int num_of_par=sizeof(par)/sizeof(cmp_seq);
+
+cmp_seq first_set[]=
+{
+	{{__left_curve,I},E},
+	{{__left_curve,I},T},
+	{{__left_curve,I},F},
+	{{__add,__sub,e},_E},
+	{{__mul,__div,e},_T},
+};
+int num_of_set=sizeof(first_set)/sizeof(cmp_seq);
+
+bool isEnd(int type)
+{
+	return ((type==__left_curve) || (type==__right_curve) || (type==I) || (type==__add) || (type==__sub) || (type==__mul) || (type==__div) || (type==e) || (type==sharp));
+}
+bool isHead(int head,int type)
+{
+	for(int i=0;i<num_of_set;++i)
+	{
+		if(type==first_set[i].res)
+		{
+			for(int j=0;j<max_len;++j)
+				if(head==first_set[i].tokens[j])
+					return true;
+		}
+	}
+	return false;
+}
+int correct_token_seq(int head,int type)
+{
+	for(int i=0;i<num_of_par;++i)
+	{
+		if((type==par[i].res) && (isHead(head,par[i].tokens[0]) || (head==par[i].tokens[0])))
+			return i;
+	}
+	for(int i=0;i<num_of_par;++i)
+	{
+		if((type==par[i].res) && (par[i].tokens[0]==e))
+			return i;
+	}
+	return -1;
+}
+int place_operator_end(int type)
+{
+	for(int i=0;i<num_of_par;++i)
+	{
+		if((type==par[i].res) && (par[i].tokens[0]==e))
+			return i;
+	}
+	return -1;
+}
+
+
+
+void print_token(int type)
+{
+	std::string str="";
+	switch(type)
+	{
+		case e:
+			str="e";
+			break;
+		case E:
+			str="E";
+			break;
+		case _E:
+			str="E\'";
+			break;
+		case T:
+			str="T";
+			break;
+		case _T:
+			str="T\'";
+			break;
+		case F:
+			str="F";
+			break;
+		case I:
+			str="i";
+			break;
+		case __add:
+			str="+";
+			break;
+		case __sub:
+			str="-";
+			break;
+		case __mul:
+			str="*";
+			break;
+		case __div:
+			str="/";
+			break;
+		case __left_curve:
+			str="(";
+			break;
+		case __right_curve:
+			str=")";
+			break;
+		case sharp:
+			str="#";
+			break;
+	}
+	std::cout<<str;
+	return;
+}
+
+class PDA
+{
+	private:
+		std::stack<int> main_stack;
+		std::stack<int> comp_stack;
+	public:
+		PDA(std::string& text)
+		{
+			main_stack.push(sharp);
+			for(int i=(int)text.length()-1;i>=0;--i)
+			{
+				if(text[i]=='(')
+					main_stack.push(__left_curve);
+				else if(text[i]==')')
+					main_stack.push(__right_curve);
+				else if(text[i]=='i')
+					main_stack.push(I);
+				else if(text[i]=='+')
+					main_stack.push(__add);
+				else if(text[i]=='-')
+					main_stack.push(__sub);
+				else if(text[i]=='*')
+					main_stack.push(__mul);
+				else if(text[i]=='/')
+					main_stack.push(__div);
+			}
+			comp_stack.push(sharp);
+			comp_stack.push(E);
+		}
+		void print_stack(std::stack<int>& temp)
+		{
+			std::stack<int> t;
+			while(!temp.empty())
+			{
+				t.push(temp.top());
+				print_token(t.top());
+				temp.pop();
+			}
+			while(!t.empty())
+			{
+				temp.push(t.top());
+				t.pop();
+			}
+			return;
+		}
+		void print_main_and_comp()
+		{
+			print_stack(main_stack);
+			std::cout<<"		";
+			print_stack(comp_stack);
+			std::cout<<std::endl;
+			return;
+		}
+		void work()
+		{
+			print_main_and_comp();
+			while((main_stack.top()!=sharp) || (comp_stack.top()!=sharp))
+			{
+				if(!isEnd(comp_stack.top()))
+				{
+					if(isHead(main_stack.top(),comp_stack.top()))
+					{
+						int i=correct_token_seq(main_stack.top(),comp_stack.top());
+						comp_stack.pop();
+						for(int j=max_len-1;j>=0;--j)
+							if(par[i].tokens[j])
+								comp_stack.push(par[i].tokens[j]);
+					}
+					else if(!isHead(main_stack.top(),comp_stack.top()))
+					{
+						//to check if this can be extended as 'e'
+						int i=place_operator_end(comp_stack.top());
+						if(i!=-1)
+							comp_stack.pop();
+						else
+						{
+							std::cout<<"error."<<std::endl;
+							return;
+						}
+					}
+					else
+					{
+						std::cout<<"error."<<std::endl;
+						return;
+					}
+					print_main_and_comp();
+				}
+				else
+				{
+					if(main_stack.top()==comp_stack.top())
+					{
+						main_stack.pop();
+						comp_stack.pop();
+					}
+					else if(comp_stack.top()==e)
+						comp_stack.pop();
+					else
+					{
+						std::cout<<"error."<<std::endl;
+						return;
+					}
+					print_main_and_comp();
+				}
+			}
+			std::cout<<"accept."<<std::endl;
+			return;
+		}
+};
+
+int main()
+{
+	std::string text;
+	while(1)
+	{
+		std::cin>>text;
+		PDA m(text);
+		m.work();
+	}
+	return 0;
+}