Fix(Core): str, txt and tex methods for MOmonial with -1 in coef

mapytex/calculus/core/MO/monomial.py

@@ -190,6 +190,27 @@ class MOMonomial(Molecule):
 
         Molecule.__init__(self, _tree)
 
+
+    def __str__(self):
+        if self._coefficient != -1:
+            return super(MOMonomial, self).__str__()
+        else:
+            return "- " + self.strpower.__str__()
+
+    @property
+    def __txt__(self):
+        if self._coefficient != -1:
+            return super(MOMonomial, self).__txt__
+        else:
+            return "- " + self.strpower.__txt__
+
+    @property
+    def __tex__(self):
+        if self._coefficient != -1:
+            return super(MOMonomial, self).__tex__
+        else:
+            return "- " + self.strpower.__tex__
+
     @property
     def coefficient(self):
         return self._coefficient