diff --git a/lark/parse_tree_builder.py b/lark/parse_tree_builder.py
index 977c371..b54b6e8 100644
--- a/lark/parse_tree_builder.py
+++ b/lark/parse_tree_builder.py
@@ -2,6 +2,7 @@ from .exceptions import GrammarError
 from .lexer import Token
 from .tree import Tree
 from .visitors import InlineTransformer # XXX Deprecated
+from .visitors import Transformer_InPlace
 
 ###{standalone
 from functools import partial, wraps
@@ -193,6 +194,14 @@ def ptb_inline_args(func):
         return func(*children)
     return f
 
+def inplace_transformer(func):
+    @wraps(func)
+    def f(children):
+        # function name in a Transformer is a rule name.
+        tree = Tree(func.__name__, children)
+        return func(tree)
+    return f
+
 class ParseTreeBuilder:
     def __init__(self, rules, tree_class, propagate_positions=False, keep_all_tokens=False, ambiguous=False, maybe_placeholders=False):
         self.tree_class = tree_class
@@ -231,6 +240,8 @@ class ParseTreeBuilder:
                 # XXX InlineTransformer is deprecated!
                 if getattr(f, 'inline', False) or isinstance(transformer, InlineTransformer):
                     f = ptb_inline_args(f)
+                elif hasattr(f, 'whole_tree') or isinstance(transformer, Transformer_InPlace):
+                    f = inplace_transformer(f)
             except AttributeError:
                 f = partial(self.tree_class, user_callback_name)
 
diff --git a/lark/parser_frontends.py b/lark/parser_frontends.py
index ab69d01..0634814 100644
--- a/lark/parser_frontends.py
+++ b/lark/parser_frontends.py
@@ -107,7 +107,7 @@ class LALR_ContextualLexer(LALR_WithLexer):
 ###}
 
 class LALR_CustomLexer(LALR_WithLexer):
-    def __init__(self, lexer_cls, lexer_conf, parser_conf, *, options=None):
+    def __init__(self, lexer_cls, lexer_conf, parser_conf, options=None):
         self.lexer = lexer_cls(self.lexer_conf)
         debug = options.debug if options else False
         self.parser = LALR_Parser(parser_conf, debug=debug)
diff --git a/lark/visitors.py b/lark/visitors.py
index 53847f9..4a0f639 100644
--- a/lark/visitors.py
+++ b/lark/visitors.py
@@ -36,7 +36,7 @@ class Transformer:
                     return f(*children)
                 elif getattr(f, 'whole_tree', False):
                     if new_children is not None:
-                        raise NotImplementedError("Doesn't work with the base Transformer class")
+                        tree.children = new_children
                     return f(tree)
                 else:
                     return f(children)
diff --git a/tests/test_parser.py b/tests/test_parser.py
index ce8b7d6..1cf702d 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -20,7 +20,7 @@ logging.basicConfig(level=logging.INFO)
 from lark.lark import Lark
 from lark.exceptions import GrammarError, ParseError, UnexpectedToken, UnexpectedInput, UnexpectedCharacters
 from lark.tree import Tree
-from lark.visitors import Transformer
+from lark.visitors import Transformer, Transformer_InPlace, v_args
 from lark.grammar import Rule
 from lark.lexer import TerminalDef
 
@@ -150,6 +150,51 @@ class TestParsers(unittest.TestCase):
         r = g.parse("xx")
         self.assertEqual( r.children, ["<c>"] )
 
+    def test_embedded_transformer_inplace(self):
+        @v_args(tree=True)
+        class T1(Transformer_InPlace):
+            def a(self, tree):
+                assert isinstance(tree, Tree), tree
+                tree.children.append("tested")
+                return tree
+
+            def b(self, tree):
+                return Tree(tree.data, tree.children + ['tested2'])
+
+        @v_args(tree=True)
+        class T2(Transformer):
+            def a(self, tree):
+                assert isinstance(tree, Tree)
+                tree.children.append("tested")
+                return tree
+
+            def b(self, tree):
+                return Tree(tree.data, tree.children + ['tested2'])
+
+        class T3(Transformer):
+            @v_args(tree=True)
+            def a(self, tree):
+                assert isinstance(tree, Tree)
+                tree.children.append("tested")
+                return tree
+
+            @v_args(tree=True)
+            def b(self, tree):
+                return Tree(tree.data, tree.children + ['tested2'])
+
+        for t in [T1(), T2(), T3()]:
+            for internal in [False, True]:
+                g = Lark("""start: a b
+                            a : "x"
+                            b : "y"
+                        """, parser='lalr', transformer=t if internal else None)
+                r = g.parse("xy")
+                if not internal:
+                    r = t.transform(r)
+
+                a, b = r.children
+                self.assertEqual(a.children, ["tested"])
+                self.assertEqual(b.children, ["tested2"])
 
     def test_alias(self):
         Lark("""start: ["a"] "b" ["c"] "e" ["f"] ["g"] ["h"] "x" -> d """)