diff --git a/balloon/abstract_syntax_tree.cpp b/balloon/abstract_syntax_tree.cpp
index f0ffea6..56bbb3a 100644
--- a/balloon/abstract_syntax_tree.cpp
+++ b/balloon/abstract_syntax_tree.cpp
@@ -478,11 +478,20 @@ var abstract_syntax_tree::call_identifier()
 			{
 				var place=i->children.front().calculation();
 				if(place.get_type()==__var_number)
-					temp=temp.get_array_member((int)place.get_number());
+				{
+					if(place.get_number()>=0)
+						temp=temp.get_array_member((int)place.get_number());
+					else
+					{
+						exit_type=__get_value_failure;
+						std::cout<<">>[Runtime-error] line"<<i->line<<": number shouldn't be less than 0 when calling an array."<<std::endl;
+						break;
+					}
+				}
 				else
 				{
 					exit_type=__error_value_type;
-					std::cout<<">>[Runtime-error] line "<<line<<": ";
+					std::cout<<">>[Runtime-error] line "<<i->line<<": ";
 					print_detail_token(i->type);
 					std::cout<<": incorrect type \'";
 					print_scalar(temp.get_type());
@@ -496,7 +505,7 @@ var abstract_syntax_tree::call_identifier()
 				if(temp.get_type()==__null_type)
 				{
 					exit_type=__get_value_failure;
-					std::cout<<">>[Runtime-error] line "<<line<<": cannot find a hash-member named \'"<<i->name<<"\' or this value is set to __null_type. detail: ";
+					std::cout<<">>[Runtime-error] line "<<i->line<<": cannot find a hash-member named \'"<<i->name<<"\' or this value is set to __null_type. detail: ";
 					temp.print_var();
 					std::cout<<"."<<std::endl;
 					break;
@@ -516,7 +525,7 @@ var abstract_syntax_tree::call_identifier()
 			else
 			{
 				exit_type=__error_value_type;
-				std::cout<<">>[Runtime-error] line "<<line<<": ";
+				std::cout<<">>[Runtime-error] line "<<i->line<<": ";
 				print_detail_token(i->type);
 				std::cout<<": incorrect type \'";
 				print_scalar(temp.get_type());
@@ -555,11 +564,20 @@ var* abstract_syntax_tree::get_var_addr()
 			{
 				var place=i->children.front().calculation();
 				if(place.get_type()==__var_number)
-					addr=addr->get_array_member_addr((int)place.get_number());
+				{
+					if(place.get_number()>=0)
+						addr=addr->get_array_member_addr((int)place.get_number());
+					else
+					{
+						exit_type=__get_value_failure;
+						std::cout<<">>[Runtime-error] line"<<i->line<<": number shouldn't be less than 0 when calling an array."<<std::endl;
+						break;
+					}
+				}
 				else
 				{
 					exit_type=__error_value_type;
-					std::cout<<">>[Runtime-error] line "<<line<<": ";
+					std::cout<<">>[Runtime-error] line "<<i->line<<": ";
 					print_detail_token(i->type);
 					std::cout<<": incorrect type \'";
 					print_scalar(addr->get_type());
@@ -573,20 +591,20 @@ var* abstract_syntax_tree::get_var_addr()
 				if(addr->get_type()==__null_type)
 				{
 					exit_type=__get_value_failure;
-					std::cout<<">>[Runtime-error] line "<<line<<": cannot find a hash-member named \'"<<i->name<<"\'."<<std::endl;
+					std::cout<<">>[Runtime-error] line "<<i->line<<": cannot find a hash-member named \'"<<i->name<<"\'."<<std::endl;
 					break;
 				}
 			}
 			else if(i->type==__call_function && addr->get_type()==__var_function)
 			{
 				exit_type=__error_value_type;
-				std::cout<<">>[Runtime-error] line "<<line<<": function-returned value cannot be assigned."<<std::endl;
+				std::cout<<">>[Runtime-error] line "<<i->line<<": function-returned value cannot be assigned."<<std::endl;
 				break;
 			}
 			else
 			{
 				exit_type=__error_value_type;
-				std::cout<<">>[Runtime-error] line "<<line<<": ";
+				std::cout<<">>[Runtime-error] line "<<i->line<<": ";
 				print_detail_token(i->type);
 				std::cout<<": incorrect type \'";
 				print_scalar(addr->get_type());
diff --git a/balloon/balloon_lexer.h b/balloon/balloon_lexer.h
index a2556cb..d286a2f 100644
--- a/balloon/balloon_lexer.h
+++ b/balloon/balloon_lexer.h
@@ -5,16 +5,16 @@
 	reserve words are those below
 	and they are also the reserve words of nasal
 */
-std::string reserve_word[15]=
+std::string reserve_word[14]=
 {
 	"for","foreach","forindex","while",
 	"var","func","break","continue","return",
-	"if","else","elsif","nil","and","or"
+	"if","else","elsif","and","or"
 };
 
 int is_reserve_word(std::string str)
 {
-	for(int i=0;i<15;++i)
+	for(int i=0;i<14;++i)
 		if(reserve_word[i]==str)
 			return __reserve_word;
 	return __token_identifier;
@@ -111,6 +111,11 @@ class resource_file
 				std::cout<<">>[Resource] lack lib file: lib/io.nas ."<<std::endl;
 			else
 				input_lib_file(lib_name);
+			lib_name="lib/basics.nas";
+			if(access("lib/basics.nas",0))
+				std::cout<<">>[Resource] lack lib file: lib/basics.nas ."<<std::endl;
+			else
+				input_lib_file(lib_name);
 			return;
 		}
 		void input_lib_file(std::string filename)
diff --git a/balloon/lib/basics.nas b/balloon/lib/basics.nas
new file mode 100644
index 0000000..98eb592
--- /dev/null
+++ b/balloon/lib/basics.nas
@@ -0,0 +1,22 @@
+# this file includes functions:
+#   append
+#   subvec
+var append(vector,elements...)
+{
+    return __call_special_inline_function(vector,elements);
+};
+
+var subvec(vector,begin,length)
+{
+    var new_vector=[];
+    for(var i=begin;i<begin+length;i+=1)
+    {
+        append(new_vector,vector[i]);
+    }
+    return new_vector;
+};
+
+var int(value)
+{
+    return __call_Cpp_int(value);
+};
\ No newline at end of file
diff --git a/balloon/lib/io.nas b/balloon/lib/io.nas
index ec6ea50..ef1e332 100644
--- a/balloon/lib/io.nas
+++ b/balloon/lib/io.nas
@@ -1,7 +1,14 @@
 
 # the function print is written by Cpp: std::cout
-# so the code is fake XD but the definition is of great use
+# the function input is written by Cpp: std::cin
+# so the code is fake XD but the definitions are of great use
+# the definitions are set to avoid redefinition
 var print=func(args...)
 {
     return __call_cpp_ostream_std_cout(args);
+};
+
+var input=func()
+{
+    return __call_cpp_istream_std_cin();
 };
\ No newline at end of file