diff --git a/mapytex/calculus/API/expression.py b/mapytex/calculus/API/expression.py
index 5786db1..b026a55 100644
--- a/mapytex/calculus/API/expression.py
+++ b/mapytex/calculus/API/expression.py
@@ -10,6 +10,7 @@
 Expression
 
 """
+from ..core import Tree, compute, tree2txt, tree2tex
 
 
 
@@ -18,12 +19,25 @@ class Expression(object):
     """
     Expression class
 
+    :example:
+
+    >>> e = Expression.from_str("2+3*4")
+    >>> e2 = e.simplify()
+    >>> print(e2)
+    14
+    >>> e2.explain()
+    '2 + 3 * 4'
+    '2 + 12'
+    '15'
+
     """
 
-    def __init__(self,):
+    RENDER = tree2txt
+
+    def __init__(self, tree):
         """
         """
-        pass
+        self._tree = tree
 
     @classmethod
     def from_str(cls,  string):
@@ -33,7 +47,8 @@ class Expression(object):
         :returns: TODO
 
         """
-        pass
+        t = Tree.from_str(string)
+        return cls(t)
 
     @classmethod
     def random(self, template, conditions = [], shuffle = False):
@@ -47,6 +62,27 @@ class Expression(object):
         """
         pass
 
+    @classmethod
+    def set_render(cls, render):
+        """ Define default render function 
+
+        :param render: render function Tree -> str
+
+        :example:
+        >>> e = Expression.from_str("2+3*4")
+        >>> print(e)
+        2 + 3 * 4
+        >>> Expression.set_render(tree2tex)
+        >>> print(e)
+        2 + 3 \\times 4
+        """
+        cls.RENDER = render
+
+    def __str__(self):
+        return self.__class__.RENDER(self._tree)
+
+    def __repr__(self):
+        return f"<Tree {self.__class__.RENDER(self._tree)}>"
 
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