Feat(Core): Change default str function for MO

mapytex/calculus/core/renders/tree2txt.py

@@ -84,6 +84,8 @@ def mul2txt(left, right):
     >>> a = MO.factory('x')
     >>> mul2txt(MO.factory(3), a)
     '3x'
+    >>> mul2txt(MO.factory(-3), a)
+    '- 3x'
     >>> mul2txt(a, a)
     'x * x'
     """
@@ -92,8 +94,11 @@ def mul2txt(left, right):
     left_ = render_with_parenthesis(left, "*")
     right_ = render_with_parenthesis(right, "*")
 
-    if (right_[0].isalpha() and (left_.isnumeric() or left_.isdecimal())) or \
-            right_[0] == '(':
+    if right_[0].isalpha():
+        # TODO: C'est bien beurk en dessous... |ven. déc. 21 12:03:07 CET 2018
+        if type(left).__name__ == 'MOnumber':
+            display_time = False
+    elif right_[0] == '(':
         display_time = False
 
     if display_time: