diff --git a/mapytex/calculus/API/expression.py b/mapytex/calculus/API/expression.py
index b876ff3..0b2d833 100644
--- a/mapytex/calculus/API/expression.py
+++ b/mapytex/calculus/API/expression.py
@@ -461,6 +461,8 @@ class Expression(object):
         <Quadratic 3n^2 + 2n + 1>
         >>> f("u_n")
         <Exp: 3u_n^2 + 2u_n + 1>
+        >>> f(f)
+        <Polynomial 36x^2 + 16x + 6 + 36x^3 + 27x^4>
         """
         tree = self._tree
         variable = (set(tree.get_leafs(extract_variable)) - {None}).pop()