diff --git a/lark/load_grammar.py b/lark/load_grammar.py
index f2f5499..7c64196 100644
--- a/lark/load_grammar.py
+++ b/lark/load_grammar.py
@@ -358,9 +358,8 @@ class SimplifyRule_Visitor(Visitor):
 
     @staticmethod
     def _flatten(tree):
-        while True:
-            if not tree.expand_kids_by_data(tree.data):
-                break
+        while tree.expand_kids_by_data(tree.data):
+            pass
 
     def expansion(self, tree):
         # rules_list unpacking
diff --git a/lark/tree.py b/lark/tree.py
index 8a29bcb..1d14bf3 100644
--- a/lark/tree.py
+++ b/lark/tree.py
@@ -110,12 +110,14 @@ class Tree(object):
     def expand_kids_by_data(self, *data_values):
         """Expand (inline) children with any of the given data values. Returns True if anything changed"""
         indices = [i for i, c in enumerate(self.children) if isinstance(c, Tree) and c.data in data_values]
-        if indices:
-            self.expand_kids_by_index(*indices)
-            return True
-        else:
+        if not indices:
             return False
 
+        for i in reversed(indices):  # reverse so that changing tail won't affect indices
+            child = self.children[i]
+            self.children[i:i+1] = child.children
+        return True
+
     def scan_values(self, pred):
         """Return all values in the tree that evaluate pred(value) as true.