diff --git a/pymath/expression.py b/pymath/expression.py
index e48f460..8b08f42 100644
--- a/pymath/expression.py
+++ b/pymath/expression.py
@@ -1,10 +1,10 @@
 #!/usr/bin/env python
 # encoding: utf-8
 
-from generic import Stack, flatten_list, expand_list
-from fraction import Fraction
-from renders import txt_render, post2in_fix, tex_render
-from formal import FormalExp
+from .generic import Stack, flatten_list, expand_list
+from .fraction import Fraction
+from .render import txt_render, post2in_fix, tex_render
+from .formal import FormalExp
 
 class Expression(object):
     """A calculus expression. Today it can andle only expression with numbers later it will be able to manipulate unknown"""
@@ -324,11 +324,7 @@ class Expression(object):
         """
         return type(exp) == int or \
                 type(exp) == Fraction or \
-<<<<<<< HEAD
                 type(exp) == FormalExp
-=======
-                exp.isalpha()
->>>>>>> Render ~work with letters still some bugs
 
     @staticmethod
     def isOperator(exp):
@@ -354,6 +350,9 @@ if __name__ == '__main__':
     #exp = "2 ^ 3 * 5"
     #test(exp)
 
+    #exp = "1 + 3 * 5"
+    #test(exp)
+
     #exp = "2 * 3 * 3 * 5"
     #test(exp)
 
@@ -404,15 +403,22 @@ if __name__ == '__main__':
     #exp="-2*b+a(12 + 1)(3-12)"
     #test(exp)
 
+    #exp="(-2+5)/(3*4)+1/12+5*5"
+    #test(exp)
+
     # TODO: The next one doesn't work  |ven. janv. 17 14:56:58 CET 2014
     #exp="-2*(-a)(12 + 1)(3-12)"
     #e = Expression(exp)
     #print(e)
 
-    exp="-2*a(12 + 1)(3-12)"
+    exp="-2+a+(12 + 1)(3-12) + 34a"
+    test(exp)
     e = Expression(exp)
     print(e)
 
+    #exp="-2*b+a(12 + 1)(3-12)"
+    #test(exp)
+
     # TODO: The next one doesn't work  |ven. janv. 17 14:56:58 CET 2014
     #exp="-2*(-a)(12 + 1)(3-12)"
     #e = Expression(exp)
diff --git a/pymath/formal.py b/pymath/formal.py
new file mode 100644
index 0000000..5901fb7
--- /dev/null
+++ b/pymath/formal.py
@@ -0,0 +1,128 @@
+#!/usr/bin/env python
+# encoding: utf-8
+
+from .fraction import Fraction
+from .generic import add_in_dict, remove_in_dict
+import re
+
+class FormalExp(object):
+    """A formal expression (similare to Symbol in Sympy"""
+
+    def __init__(self, coef = {}, letter = ""):
+        """Initiat the formal expression
+
+        :param coef: the dictionary representing the expression
+        :param letter:  minimum expression, a letter 
+
+        """
+
+        if coef != {} and letter != "":
+            raise ValueError("A FormalExp can't be initiate with dict_exp and a letter")
+        elif letter != "":
+            self._letter = letter
+            self._coef = {letter: 1}
+        elif coef != {}:
+            self._coef = coef
+        else:
+            raise ValueError("FormalExp needs a letter or dictionary of coeficients")
+
+        if len(self) != 1:
+            self.mainOp = "+"
+
+    def master_coef(self):
+        """Return the master coefficient
+        /!\ may not work pretty well if there is more than one indeterminate
+        :returns: a_n
+
+        """
+        pattern = "\w\*\*(\d*)"
+        finder = re.compile(pattern)
+        power = {}
+        for (k,v) in self._coef.items():
+            if k=="":
+                power[0] = v
+            else:
+                p = finder.findall(k)
+                if p == []:
+                    power[1] = v
+                else:
+                    power[int(p[0])] =  v
+
+        m_power = max(power)
+        return power[m_power]
+
+    def __add__(self, other):
+        if type(other) in [int, Fraction]:
+            d = {"":other}
+        elif type(other) == FormalExp:
+            d = other._coef
+        else:
+            raise ValueError("Can't add {type} with FormalExp".format(type=type(other)))
+
+        d = add_in_dict(self._coef, d)
+        d = remove_in_dict(d)
+        if list(d.keys()) == ['']:
+            return [d['']]
+        else:
+            return [FormalExp(d)]
+
+    def __radd__(self, other):
+        return self + other
+
+    def __sub__(self, other):
+        o_tmp = -other
+        return self + o_tmp
+
+    def __neg__(self):
+        d = {}
+        for k,v in self._coef.items():
+            d[k] = -v
+        return FormalExp(d)    
+
+    def __mul__(self, other):
+        pass
+    
+    def __rmul__(self, other):
+        pass
+
+    def __div__(self, other):
+        pass
+    
+    def __pow__(self, other):
+        pass
+
+    def __len__(self):
+        return len(list(self._coef.keys()))
+
+    def __str__(self):
+        return " + ".join([str(v) + str(k) for k,v in self._coef.items()])
+    
+if __name__ == '__main__':
+    #fe1 = FormalExp({"x": 1, "":2})    
+    #print(fe1)
+    #fe2 = FormalExp({"x**12": 5, "":2})    
+    #print(fe2)
+    #fe3 = fe1 + fe2
+    #for s in fe3:
+    #    print(s)
+    #fe4 = fe1 + 2
+    #for s in fe4:
+    #    print(s)
+
+    #print(fe1.master_coef())
+    #print(fe2.master_coef())
+    #print(fe3[0].master_coef())
+    #print(fe4[0].master_coef())
+
+    fe = FormalExp(letter = "a")
+    fe_ = -2 + fe
+    print(fe_[0])
+
+
+
+
+
+# -----------------------------
+# Reglages pour 'vim'
+# vim:set autoindent expandtab tabstop=4 shiftwidth=4:
+# cursor: 16 del 
diff --git a/pymath/render.py b/pymath/render.py
index 326224b..9ba3d35 100644
--- a/pymath/render.py
+++ b/pymath/render.py
@@ -1,10 +1,11 @@
 #!/usr/bin/env python
 # encoding: utf-8
 
-from generic import Stack,flatten_list
-from fraction import Fraction
-from formal import FormalExp
+from .generic import Stack,flatten_list
+from .fraction import Fraction
+from .formal import FormalExp
 
+        
 class Render(object):
     """A class which aims to create render functions from three dictionnaries:
         - op_infix: dict of caracters
@@ -15,7 +16,7 @@ class Render(object):
 
     PRIORITY = {"^": 4,"*" : 3, "/": 3, ":": 3, "+": 2, "-":2, "(": 1}
 
-    def __init__(self, op_infix = {}, op_postfix = {}, other = {}, join = " ", type_render = {int: str, Fraction: str, str: str}):
+    def __init__(self, op_infix = {}, op_postfix = {}, other = {}, join = " ", type_render = {int: str, Fraction: str, FormalExp: str}):
         """Initiate the render
         
         @param op_infix: the dictionnary of infix operator with how they have to be render