diff --git a/mapytex/__init__.py b/mapytex/__init__.py
index 691d8cc..57e3ee5 100644
--- a/mapytex/__init__.py
+++ b/mapytex/__init__.py
@@ -3,7 +3,7 @@
 
 from .calculus import Expression#, Polynom, Fraction, random_str, txt, Equation
 
-Expression.RENDER = 'tex'
+Expression.set_render('tex')
 
 from .stat import Dataset, WeightedDataset
 from .geometry import random_pythagore
diff --git a/mapytex/calculus/API/__init__.py b/mapytex/calculus/API/__init__.py
index 7b77ce3..30bb26f 100644
--- a/mapytex/calculus/API/__init__.py
+++ b/mapytex/calculus/API/__init__.py
@@ -11,6 +11,7 @@ Generate and compute like a student!
 
 :example:
 
+>>> Expression.set_render("txt")
 >>> e = Expression.from_str("2+3*4")
 >>> e_simplified = e.simplify()
 >>> print(e_simplified)
diff --git a/mapytex/calculus/API/expression.py b/mapytex/calculus/API/expression.py
index 5b42b47..c207636 100644
--- a/mapytex/calculus/API/expression.py
+++ b/mapytex/calculus/API/expression.py
@@ -13,7 +13,7 @@ Expression
 from ..core import AssocialTree, Tree, compute, typing, TypingError
 from ..core.random import extract_rdleaf, extract_rv, random_generator, compute_leafs, replace_rdleaf
 from ..core.MO import moify
-from .tokens import factory
+from .tokens import factory, Token
 from .renders import renders
 
 
@@ -24,6 +24,7 @@ class Expression(object):
 
     :example:
 
+    >>> Expression.set_render("txt")
     >>> e = Expression.from_str("2+3*4")
     >>> e2 = e.simplify()
     >>> print(e2)
@@ -43,6 +44,40 @@ class Expression(object):
         self._tree = tree
         self._ancestor = ancestor
 
+    @classmethod
+    def set_render(cls, render):
+        """ Define default render function 
+
+        :param render: render name (txt or tex)
+
+        :example:
+        >>> Expression.set_render('txt')
+        >>> e = Expression.from_str("2+3*4")
+        >>> print(e)
+        2 + 3 * 4
+        >>> e = Expression.from_str("2+3/4")
+        >>> print(e)
+        2 + 3 / 4
+        >>> es = e.simplify()
+        >>> print(es)
+        11 / 4
+        >>> Expression.set_render('tex')
+        >>> Expression.RENDER
+        'tex'
+        >>> e = Expression.from_str("2+3*4")
+        >>> print(e)
+        2 + 3 \\times 4
+        >>> e = Expression.from_str("2+3/4")
+        >>> print(e)
+        2 + \\frac{3}{4}
+        >>> es = e.simplify()
+        >>> print(es)
+        \\frac{11}{4}
+        >>> Expression.set_render('txt')
+        """
+        Token.set_render(render)
+        cls.RENDER = render
+
     @classmethod
     def from_str(cls, string, typing=True):
         """ Initiate the expression from a string
@@ -99,10 +134,12 @@ class Expression(object):
         <Exp: -3 / -15>
         >>> Expression.random("{a}/{a*k} - 3*{b}", variables_scope={'a':{'min_max':(10, 30)}}) # doctest: +SKIP
         <Exp: 18 / 108 - 3 * 9>
-        >>> e = Expression.random("{a}*x + {b}*x + 3")
+        >>> e = Expression.random("{a}*x + {b}*x + 3", ["a>b"], rejected=[0, 1])
         >>> ee = e.simplify()
         >>> print(e) # doctest: +SKIP
+        10x - 6x + 3
         >>> print(ee) # doctest: +SKIP
+        4x + 3
 
         """
         rd_t = Tree.from_str(template, random=True)
@@ -128,27 +165,6 @@ class Expression(object):
         except TypeError as e:
             return cls(t)
 
-    @classmethod
-    def set_render(cls, render):
-        """ Define default render function 
-
-        :param render: render function Tree -> str
-
-        :example:
-        >>> Expression.RENDER
-        'txt'
-        >>> e = Expression.from_str("2+3*4")
-        >>> print(e)
-        2 + 3 * 4
-        >>> Expression.set_render('tex')
-        >>> Expression.RENDER
-        'tex'
-        >>> print(e)
-        2 + 3 \\times 4
-        >>> Expression.set_render('txt')
-        """
-        cls.RENDER = render
-
     def __str__(self):
         return renders[self.RENDER](self._tree)
 
diff --git a/mapytex/calculus/API/tokens/__init__.py b/mapytex/calculus/API/tokens/__init__.py
index 2d83125..49b796b 100644
--- a/mapytex/calculus/API/tokens/__init__.py
+++ b/mapytex/calculus/API/tokens/__init__.py
@@ -19,6 +19,8 @@ from decimal import Decimal as _Decimal
 from .number import Integer, Decimal, Fraction
 from .polynomial import Polynomial, Linear, Quadratic
 
+from .token import Token
+
 __all__ = ["factory"]
 
 
diff --git a/mapytex/calculus/API/tokens/token.py b/mapytex/calculus/API/tokens/token.py
index 63871ff..1a343f2 100644
--- a/mapytex/calculus/API/tokens/token.py
+++ b/mapytex/calculus/API/tokens/token.py
@@ -29,6 +29,15 @@ class Token(object):
     def random(cls):
         raise NotImplemented
 
+
+    @classmethod
+    def set_render(cls, render):
+        """ Define default render function
+
+        :param render: render name (txt or tex)
+        """
+        cls.RENDER = render
+
     def explain(self):
         """ Yield every calculus step which have lead to self
         
diff --git a/mapytex/calculus/__init__.py b/mapytex/calculus/__init__.py
index 3914e98..91f40c9 100644
--- a/mapytex/calculus/__init__.py
+++ b/mapytex/calculus/__init__.py
@@ -13,6 +13,7 @@ Make calculus as a student
 Expression is the classe wich handle all calculus. It can randomly generate or import calculus, simplify them and explain them as a student would do.
 
 >>> from mapytex.calculus import Expression
+>>> Expression.set_render("txt")
 >>> e = Expression.from_str("2x + 6 - 3x")
 >>> print(e)
 2x + 6 - 3x
@@ -31,7 +32,7 @@ Expression is the classe wich handle all calculus. It can randomly generate or i
 
 from .API import Expression
 
-__all__ = [Expression]
+__all__ = ["Expression"]
 
 
 # -----------------------------