add a doctest for Tree

mapytex/calculus/core/tree.py

@@ -76,6 +76,14 @@ class Tree(object):
          | > 2
          | > 3
          > 4
+        >>> t = Tree.from_str("2+3*n")
+        >>> print(t)
+        +
+         > 2
+         > *
+         | > 3
+         | > n
+
         """
         t = MutableTree.from_str(expression)
         return cls.from_any_tree(t)