diff --git a/mapytex/calculus/core/tree.py b/mapytex/calculus/core/tree.py
index 013d449..f2b99c9 100644
--- a/mapytex/calculus/core/tree.py
+++ b/mapytex/calculus/core/tree.py
@@ -48,10 +48,8 @@ class Tree(object):
         >>> t.right_value
         2
         """
-        if node is None or \
-                left_value is None or \
-                right_value is None:
-                    raise TypeError("Tree can't have empty node or leaf")
+        if node is None or right_value is None:
+            raise TypeError("Tree can't have an empty node or an empty right leaf")
 
         self.node = node
 
@@ -64,7 +62,7 @@ class Tree(object):
 
         :example:
 
-            >>> t = MutableTree.fromStr("2+3*4")
+        >>> t = MutableTree.fromStr("2+3*4")
         >>> print(t)
         +
          > 2
@@ -492,7 +490,7 @@ class MutableTree(Tree):
                         try:
                             ans.set_node(c)
                         except ValueError:
-                            if OPERATORS[c]["priority"] > OPERATORS[ans.node]["priority"]:
+                            if OPERATORS[c]["precedence"] > OPERATORS[ans.node]["precedence"]:
                                 ans.append_bot(c)
                             else:
                                 ans.append_top(c)