fix error in stop_errors and add debug term output

headers/errors.h

@@ -11,6 +11,6 @@ typedef enum
     ERROR_PARSING = -3,
 } program_error;
 
-int stop_errors(int err, char *details);
+int stop_errors(int err, const char *details);
 
 #endif

headers/parser.h

@@ -10,12 +10,14 @@ typedef enum
 {
     TERM_LEFT,  // a in "a = b"
     TERM_RIGHT, // b in "a = b"
+    TERM_END,   // last term
 } term_position;
 
 typedef enum
 {
     TERM_PLUS,  // +
     TERM_MINUS, // -
+    TERM_NULL,  // null -> for the last term
 } term_sign;
 
 typedef struct