diff --git a/lark/parse_tree_builder.py b/lark/parse_tree_builder.py
index e0041b9..de88cd1 100644
--- a/lark/parse_tree_builder.py
+++ b/lark/parse_tree_builder.py
@@ -1,3 +1,5 @@
+from functools import partial
+
 from .common import is_terminal, GrammarError
 from .utils import suppress
 from .lexer import Token
@@ -5,15 +7,7 @@ from .grammar import Rule
 
 ###{standalone
 
-class NodeBuilder:
-    def __init__(self, tree_class, name):
-        self.tree_class = tree_class
-        self.name = name
-
-    def __call__(self, children):
-        return self.tree_class(self.name, children)
-
-class Expand1:
+class ExpandSingleChild:
     def __init__(self, node_builder):
         self.node_builder = node_builder
 
@@ -23,57 +17,17 @@ class Expand1:
         else:
             return self.node_builder(children)
 
-class Factory:
-    def __init__(self, cls, *args):
-        self.cls = cls
-        self.args = args
-
-    def __call__(self, node_builder):
-        return self.cls(node_builder, *self.args)
 
-
-class TokenWrapper:
+class CreateToken:
     "Used for fixing the results of scanless parsing"
 
-    def __init__(self, node_builder, token_name):
+    def __init__(self, token_name, node_builder):
         self.node_builder = node_builder
         self.token_name = token_name
 
     def __call__(self, children):
         return self.node_builder( [Token(self.token_name, ''.join(children))] )
 
-def identity(node_builder):
-    return node_builder
-
-
-class ChildFilter:
-    def __init__(self, node_builder, to_include):
-        self.node_builder = node_builder
-        self.to_include = to_include
-
-    def __call__(self, children):
-        filtered = []
-        for i, to_expand in self.to_include:
-            if to_expand:
-                filtered += children[i].children
-            else:
-                filtered.append(children[i])
-
-        return self.node_builder(filtered)
-
-def create_rule_handler(expansion, keep_all_tokens, filter_out):
-    # if not keep_all_tokens:
-    to_include = [(i, not is_terminal(sym) and sym.startswith('_'))
-                  for i, sym in enumerate(expansion)
-                  if keep_all_tokens
-                  or not ((is_terminal(sym) and sym.startswith('_')) or sym in filter_out)
-                  ]
-
-    if len(to_include) < len(expansion) or any(to_expand for i, to_expand in to_include):
-        return Factory(ChildFilter, to_include)
-
-    # else, if no filtering required..
-    return identity
 
 class PropagatePositions:
     def __init__(self, node_builder):
@@ -98,6 +52,31 @@ class PropagatePositions:
         return res
 
 
+class ChildFilter:
+    def __init__(self, to_include, node_builder):
+        self.node_builder = node_builder
+        self.to_include = to_include
+
+    def __call__(self, children):
+        filtered = []
+        for i, to_expand in self.to_include:
+            if to_expand:
+                filtered += children[i].children
+            else:
+                filtered.append(children[i])
+
+        return self.node_builder(filtered)
+
+def _should_expand(sym):
+    return not is_terminal(sym) and sym.startswith('_')
+
+def maybe_create_child_filter(expansion, filter_out):
+    to_include = [(i, _should_expand(sym)) for i, sym in enumerate(expansion) if sym not in filter_out]
+
+    if len(to_include) < len(expansion) or any(to_expand for i, to_expand in to_include):
+        return partial(ChildFilter, to_include)
+
+
 class Callback(object):
     pass
 
@@ -112,22 +91,20 @@ class ParseTreeBuilder:
         self.user_aliases = {}
 
     def _init_builders(self, rules):
-        filter_out = set()
-        for rule in rules:
-            if rule.options and rule.options.filter_out:
-                assert rule.origin.startswith('_')   # Just to make sure
-                filter_out.add(rule.origin)
+        filter_out = {rule.origin for rule in rules if rule.options and rule.options.filter_out}
+        filter_out |= {sym for rule in rules for sym in rule.expansion if is_terminal(sym) and sym.startswith('_')}
+        assert all(x.startswith('_') for x in filter_out)
 
         for rule in rules:
             options = rule.options
             keep_all_tokens = self.always_keep_all_tokens or (options.keep_all_tokens if options else False)
-            expand1 = options.expand1 if options else False
+            expand_single_child = options.expand1 if options else False
             create_token = options.create_token if options else False
 
             wrapper_chain = filter(None, [
-                (expand1 and not rule.alias) and Expand1,
-                create_token and Factory(TokenWrapper, create_token),
-                create_rule_handler(rule.expansion, keep_all_tokens, filter_out),
+                create_token and partial(CreateToken, create_token),
+                (expand_single_child and not rule.alias) and ExpandSingleChild,
+                maybe_create_child_filter(rule.expansion, () if keep_all_tokens else filter_out),
                 self.propagate_positions and PropagatePositions,
             ])
 
@@ -144,7 +121,7 @@ class ParseTreeBuilder:
             try:
                 f = transformer._get_func(user_callback_name)
             except AttributeError:
-                f = NodeBuilder(self.tree_class, user_callback_name)
+                f = partial(self.tree_class, user_callback_name)
 
             self.user_aliases[rule] = rule.alias
             rule.alias = internal_callback_name