Fix: add special case to add typing

Sometimes, typing/add.py tried to build MOpolynomial with only one
coefficient.

mapytex/calculus/core/MO/monomial.py

@@ -132,7 +132,7 @@ class MOstrPower(Molecule):
 
 class MOMonomial(Molecule):
 
-    """ Monomial math object"""
+    """ Monomial math object : ax^n"""
 
     MAINOP = "*"
 

mapytex/calculus/core/MO/polynomial.py

@@ -18,7 +18,7 @@ __all__ = ["MOpolynomial"]
 
 class MOpolynomial(Molecule):
 
-    """ MO polynomial"""
+    """ MO polynomial: ax^n + ... + z (can't be a monomial)"""
 
     MAINOP = "+"
 
@@ -39,6 +39,14 @@ class MOpolynomial(Molecule):
         <MOpolynomial 4x^3 + 1>
         >>> MOpolynomial('x', {0: 1, 3: 1})
         <MOpolynomial x^3 + 1>
+        >>> MOpolynomial('x', [0, 0, 3])
+        Traceback (most recent call last):
+        ...
+        TypeError: A MOpolynomial can't be monomial it has to have more than one coefficient.
+        >>> MOpolynomial('x', {3: 1})
+        Traceback (most recent call last):
+        ...
+        TypeError: A MOpolynomial can't be monomial it has to have more than one coefficient.
 
         """
         _variable = MO.factory(variable)
@@ -58,6 +66,9 @@ class MOpolynomial(Molecule):
             raise TypeError("Coefs needs to be a dictionnary or a list")
         self._coefs = _coefs
 
+        if len(self._coefs) == 1:
+            raise TypeError("A MOpolynomial can't be monomial it has to have more than one coefficient.")
+
         monomials = OrderedDict()
         for deg in sorted(self._coefs.keys()):
             coef = self._coefs[deg]