tree2txt is done and doctested

mapytex/calculus/core/MO/mo.py

@@ -80,7 +80,7 @@ class MO(object):
         return f"<{self.__class__.__name__} {self.__txt__}>"
 
     def __str__(self):
-        return self.__txt__
+        return str(self.value)
 
     @property
     def __txt__(self):
@@ -154,11 +154,13 @@ class MOnumber(MO):
         >>> MOnumber(23)
         <MOnumber 23>
         >>> MOnumber(-23)
-        <MOnumber -23>
+        <MOnumber - 23>
         >>> MOnumber(23.3)
         <MOnumber 23.300000000000000710542735760100185871124267578125>
         >>> MOnumber(Decimal("23.3"))
         <MOnumber 23.3>
+        >>> MOnumber(Decimal("-23.3"))
+        <MOnumber - 23.3>
         >>> a = MOnumber(23)
         >>> MOnumber(a)
         <MOnumber 23>
@@ -180,6 +182,13 @@ class MOnumber(MO):
         else:
             raise MOError("The value of an MOnumber need to be a int, a float or a Decimal")
 
+    @property
+    def __txt__(self):
+        if self.value > 0:
+            return str(self.value)
+
+        return f"- {abs(self.value)}"
+
         
 class MOstr(MO):