diff --git a/headers/errors.h b/headers/errors.h
index 49ef8e2..0034387 100644
--- a/headers/errors.h
+++ b/headers/errors.h
@@ -1,17 +1,17 @@
 #ifndef ERRORS_H
 #define ERRORS_H
 
-#include "../libft/includes/libft.h"
-
 typedef enum
 {
-    ERROR_BASIC = 0,
-    ERROR_UNKNOWN_TOKEN = -1,
-    ERROR_NUMBER_TOO_BIG = -2,
-    ERROR_PARSING = -3,
-    ERROR_TOKEN_POSITION = -4,
+    ERROR_BASE = 1, // Start at 1 to avoid exit(0) for errors
+    ERROR_UNKNOWN_TOKEN,
+    ERROR_NUMBER_TOO_BIG,
+    ERROR_PARSING,
+    ERROR_TOKEN_POSITION,
+    ERROR_VAR_DIFF,
+    ERROR_SENTINEL,
 } program_error;
 
-int stop_errors(program_error err, const char *details);
+int stop_errors(program_error err, const char *format, ...);
 
 #endif
\ No newline at end of file
diff --git a/libft b/libft
index f86c2cf..b64ede5 160000
--- a/libft
+++ b/libft
@@ -1 +1 @@
-Subproject commit f86c2cf5cb805cb2bf863b27d42169c5b1f0ccd2
+Subproject commit b64ede50af8fd14ff7a55be6fbba2f34a7aa8a3e
diff --git a/src/errors.c b/src/errors.c
index d8a7b84..f298822 100644
--- a/src/errors.c
+++ b/src/errors.c
@@ -1,27 +1,38 @@
 #include "errors.h"
+#include "libft.h"
+#include <stdarg.h>
 
-int stop_errors(program_error err, const char *details)
+int stop_errors(program_error err, const char *details, ...)
 {
-    switch (err)
+    // the base error message
+    const char *msg = "error: error type is out of range";
+
+    // Map error codes to messages
+    const char *error_messages[] = {
+        [ERROR_BASE] = "error: undefined error, details :",
+        [ERROR_UNKNOWN_TOKEN] = "error: unknown token, details :",
+        [ERROR_NUMBER_TOO_BIG] = "error: number is too big, details :",
+        [ERROR_PARSING] = "error: too much terms to parse, details :",
+        [ERROR_TOKEN_POSITION] = "error: token position is not good in grammar, details :",
+        [ERROR_VAR_DIFF] = "error: expression must only contain one variable, details :",
+        // ⚠️ Add new error messages here when adding new error codes!
+    };
+
+    // Override msg if err is in the error_messages array
+    if (err >= ERROR_BASE && err < ERROR_SENTINEL)
     {
-    case ERROR_UNKNOWN_TOKEN:
-        ft_putstr_fd("error: unknown token, details : ", STDERR_FILENO);
-        break;
-    case ERROR_NUMBER_TOO_BIG:
-        ft_putstr_fd("error: number is too big, details : ", STDERR_FILENO);
-        break;
-    case ERROR_PARSING:
-        ft_putstr_fd("error: too much terms to parse, details : ", STDERR_FILENO);
-        break;
-    case ERROR_TOKEN_POSITION:
-        ft_putstr_fd("error: token position is not good in grammar, details : ", STDERR_FILENO);
-        break;
-    default:
-        ft_putstr_fd("unknown error, details : ", STDERR_FILENO);
-        break;
+        msg = error_messages[err];
     }
 
-    ft_putstr_fd(details, STDERR_FILENO);
+    // Print the base message
+    ft_dprintf(STDERR_FILENO, "%s (%i) - ", msg, err);
+
+    // Print the formatted details directly
+    va_list args;
+    va_start(args, details);
+    ft_vdprintf(STDERR_FILENO, details, args);
+    va_end(args);
+
     ft_putchar_fd('\n', STDERR_FILENO);
 
     exit(err);
diff --git a/src/parser.c b/src/parser.c
index 7079237..4c0f3b8 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -1,15 +1,16 @@
 #include "parser.h"
 
 /**
-    TOKEN_VARIABLE,       // x, y, etc.
-    TOKEN_NUMBER_INT,     // int
-    TOKEN_NUMBER_DOUBLE,  // double
-    TOKEN_POWER,          // ^ or **
-    TOKEN_SIGN,           // + or -
-    TOKEN_MULTIPLICATION, // *
-    TOKEN_DIVISION,       // /
-    TOKEN_EQUAL,          // =
-    TOKEN_END             // null (end of input)
+    TOKEN_VARIABLE,      // x, y, etc.
+    TOKEN_NUMBER_INT,    // int
+    TOKEN_NUMBER_DOUBLE, // double
+    TOKEN_POWER,         // ^ or **
+    TOKEN_SIGN_PLUS,     // +
+    TOKEN_SIGN_MINUS,    // -
+    TOKEN_FACTOR_MULT,   // *
+    TOKEN_FACTOR_DIV,    // / or :
+    TOKEN_EQUAL,         // =
+    TOKEN_END            // null (end of input)
 
     1.   VAR |   NUMBER_I |   NUMBER_D | ! POW |   SIGN_P |   SIGN_M | ! FACTOR_MUL | ! FACTOR_DIV | ! EQUAL | END
          NT  |   NUMBER                |   NT  |   SIGN              | ! FACTOR                    |   NT    | NT
@@ -17,8 +18,8 @@
     2.   VAR |   NUMBER_I |   NUMBER_D | ! POW | ! SIGN_P | ! SIGN_M | ! FACTOR_MUL | ! FACTOR_DIV | ! EQUAL | END
          NT  |   NUMBER                |   NT  | ! SIGN              | ! FACTOR                    |   NT    | NT
 
-    3.   VAR |   NUMBER_I |   NUMBER_D | ! POW |   SIGN_P |   SIGN_M |   FACTOR_MUL |   FACTOR_DIV |   EQUAL | END
-         NT  |   NUMBER                |   NT  |   SIGN              |   FACTOR                    |   NT    | NT
+    3.   VAR | ! NUMBER_I | ! NUMBER_D | ! POW | ! SIGN_P | ! SIGN_M |   FACTOR_MUL | ! FACTOR_DIV | ! EQUAL | END
+         NT  | ! NUMBER                |   NT  | ! SIGN              |   FACTOR                    |   NT    | NT
 
     term_position position;
     term_sign sign;
@@ -32,15 +33,15 @@ static term_sign get_sign(token *tokens, int i, int *token_count)
     // forbidden tokens
     if (tokens[i].type == TOKEN_POWER)
     {
-        stop_errors(ERROR_TOKEN_POSITION, "at begining of term, we should not have a power token : " + tokens[i].value_char);
+        stop_errors(ERROR_TOKEN_POSITION, "at sign place, we should not have a token 'power' : %c", tokens[i].value_char);
     }
     if (tokens[i].tag == TOKEN_FACTOR)
     {
-        stop_errors(ERROR_TOKEN_POSITION, "at begining of term, we should not have a factor token : " + tokens[i].value_char);
+        stop_errors(ERROR_TOKEN_POSITION, "at sign place, we should not have a token 'factor' : %c", tokens[i].value_char);
     }
     if (tokens[i].type == TOKEN_EQUAL)
     {
-        stop_errors(ERROR_TOKEN_POSITION, "at begining of term, we should not have an equal token : " + tokens[i].value_char);
+        stop_errors(ERROR_TOKEN_POSITION, "at sign place, we should not have a token 'equal' : %c", tokens[i].value_char);
     }
 
     // sign
@@ -60,7 +61,7 @@ static term_sign get_sign(token *tokens, int i, int *token_count)
         return '+';
     }
 
-    return stop_errors(ERROR_TOKEN_POSITION, "at begining of term, we should have a sign token " + tokens[i].value_char);
+    return stop_errors(ERROR_TOKEN_POSITION, "at begining of term, we should have a token 'sign' : %c", tokens[i].value_char);
 }
 
 static double get_coefficient(token *tokens, int i, int *token_count)
@@ -72,19 +73,19 @@ static double get_coefficient(token *tokens, int i, int *token_count)
     // forbidden tokens
     if (tokens[i].type == TOKEN_POWER)
     {
-        stop_errors(ERROR_TOKEN_POSITION, "at coefficient place, we should not have a power token : " + tokens[i].value_char);
+        stop_errors(ERROR_TOKEN_POSITION, "at coefficient place, we should not have a token 'power' : %c", tokens[i].value_char);
     }
     if (tokens[i].tag == TOKEN_FACTOR)
     {
-        stop_errors(ERROR_TOKEN_POSITION, "at coefficient place, we should not have a factor token : " + tokens[i].value_char);
+        stop_errors(ERROR_TOKEN_POSITION, "at coefficient place, we should not have a token 'factor' : %c", tokens[i].value_char);
     }
     if (tokens[i].type == TOKEN_EQUAL)
     {
-        stop_errors(ERROR_TOKEN_POSITION, "at coefficient place, we should not have an equal token : " + tokens[i].value_char);
+        stop_errors(ERROR_TOKEN_POSITION, "at coefficient place, we should not have a token 'equal' : %c", tokens[i].value_char);
     }
     if (tokens[i].tag == TOKEN_SIGN)
     {
-        stop_errors(ERROR_TOKEN_POSITION, "at coefficient place, we should not have a sign token : " + tokens[i].value_char);
+        stop_errors(ERROR_TOKEN_POSITION, "at coefficient place, we should not have a token 'sign' : %c", tokens[i].value_char);
     }
 
     // if not coefficient token
@@ -128,9 +129,41 @@ static double get_coefficient(token *tokens, int i, int *token_count)
 
 static int get_exponent(token *tokens, int i, int *token_count)
 {
-    if (tokens[i].type)   // placeholder
-        *token_count = 1; // placeholder
-    return 1;             // placeholder
+    /**
+     * power
+     * number
+     * sign
+     * equal
+     * factor_div
+     */
+    // forbidden tokens
+    if (tokens[i].type == TOKEN_POWER)
+    {
+        stop_errors(ERROR_TOKEN_POSITION, "at exponent place, we should not have a token 'power' : %c", tokens[i].value_char);
+    }
+    if (tokens[i].tag == TOKEN_NUMBER)
+    {
+        stop_errors(ERROR_TOKEN_POSITION, "at exponent place, we should not have a token 'number' : %c", tokens[i].value_char);
+    }
+    if (tokens[i].tag == TOKEN_SIGN)
+    {
+        stop_errors(ERROR_TOKEN_POSITION, "at exponent place, we should not have a token 'sign' : %c", tokens[i].value_char);
+    }
+    if (tokens[i].type == TOKEN_EQUAL)
+    {
+        stop_errors(ERROR_TOKEN_POSITION, "at exponent place, we should not have a token 'equal' : %c", tokens[i].value_char);
+    }
+    if (tokens[i].type == TOKEN_FACTOR_DIV)
+    {
+        stop_errors(ERROR_TOKEN_POSITION, "at exponent place, we should not have a token 'division' : %c", tokens[i].value_char);
+    }
+
+    // if 'var' -> token_count + 1
+    // else if '*' + 'var' -> token_count + 2
+
+    token_count += 0; // placeholder
+
+    return 1; // placeholder
 }
 
 int parse(token *tokens, term *terms, int terms_count_max)
@@ -139,13 +172,29 @@ int parse(token *tokens, term *terms, int terms_count_max)
     int terms_count;
     int token_count;
     term_position term_position;
+    char var;
 
     terms_count = 0;
     token_count = 0;
     i = 0;
     term_position = TERM_LEFT;
+    var = 0;
     while (tokens[i].type != TOKEN_END && terms_count < terms_count_max)
     {
+        // variable -> all variables must be the same
+        if (tokens[i].type == TOKEN_VARIABLE)
+        {
+            if (!var)
+            {
+                var = tokens[i].value_char;
+            }
+            else if (var != tokens[i].value_char)
+            {
+                stop_errors(ERROR_VAR_DIFF, "old var : '%c' - new var : '%c'", var, tokens[i].value_char);
+            }
+        }
+
+        // equal
         if (tokens[i].type == TOKEN_EQUAL)
         {
             term_position = TERM_RIGHT;