Feat: Expression call works with tokens

mapytex/calculus/API/expression.py

@@ -442,6 +442,9 @@ class Expression(object):
     def __call__(self, value):
         """ Call a Expression to evaluate itself on value 
 
+        :param value: evaluate the Expression with this value
+        :return: Expression simplified if the value is not a string with a length greater than 1.
+
         :examples:
         >>> f = Expression.from_str("3*x^2 + 2x + 1")
         >>> for s in f(2).explain():
@@ -450,14 +453,27 @@ class Expression(object):
         3 * 4 + 4 + 1
         12 + 5
         17
+        >>> f(f(2))
+        <Integer 902>
+        >>> f(17)
+        <Integer 902>
+        >>> f("n")
+        <Quadratic 3n^2 + 2n + 1>
+        >>> f("u_n")
+        <Exp: 3u_n^2 + 2u_n + 1>
         """
         tree = self._tree
         variable = (set(tree.get_leafs(extract_variable)) - {None}).pop()
 
-        dest = moify(value)
+        try:
+            dest = value._mo
+        except AttributeError:
+            dest = moify(value)
         replace_var = partial(replace, origin=variable, dest=dest)
         tree = tree.map_on_leaf(replace_var)
 
+        if isinstance(value, str) and len(value) > 1:
+            return Expression(tree)
         return Expression(tree).simplify()