diff --git a/benedict/core/clean.py b/benedict/core/clean.py
index e2e96e1..cd2f581 100644
--- a/benedict/core/clean.py
+++ b/benedict/core/clean.py
@@ -3,13 +3,18 @@
 from benedict.utils import type_util
 
 
+def _clean_item(d, key, strings, collections):
+    value = d.get(key, None)
+    if not value:
+        del_none = (value is None)
+        del_string = (strings and type_util.is_string(value))
+        del_collection = (collections and type_util.is_collection(value))
+        return any([del_none, del_string, del_collection])
+    return False
+
+
 def clean(d, strings=True, collections=True):
     keys = list(d.keys())
     for key in keys:
-        value = d.get(key, None)
-        if not value:
-            del_none = value is None
-            del_string = strings and type_util.is_string(value)
-            del_collection = collections and type_util.is_collection(value)
-            if any([del_none, del_string, del_collection]):
-                del d[key]
+        if _clean_item(d, key, strings, collections):
+            del d[key]
diff --git a/benedict/core/flatten.py b/benedict/core/flatten.py
index 4a7dccd..4c3648e 100644
--- a/benedict/core/flatten.py
+++ b/benedict/core/flatten.py
@@ -3,19 +3,24 @@
 from benedict.utils import type_util
 
 
+def _flatten_key(base_key, key, separator):
+    if base_key and separator:
+        return '{}{}{}'.format(base_key, separator, key)
+    return key
+
+
 def flatten(d, separator='_', **kwargs):
     new_dict = d.copy()
     new_dict.clear()
     keys = list(d.keys())
     base_key = kwargs.pop('base_key', '')
     for key in keys:
-        new_key = '{}{}{}'.format(
-            base_key, separator, key) if base_key and separator else key
         value = d.get(key, None)
+        new_key = _flatten_key(base_key, key, separator)
         if type_util.is_dict(value):
             new_value = flatten(value, separator=separator, base_key=new_key)
             new_value.update(new_dict)
             new_dict.update(new_value)
-        else:
-            new_dict[new_key] = value
+            continue
+        new_dict[new_key] = value
     return new_dict
diff --git a/benedict/core/merge.py b/benedict/core/merge.py
index 87c18a7..2d92b83 100644
--- a/benedict/core/merge.py
+++ b/benedict/core/merge.py
@@ -3,13 +3,21 @@
 from benedict.utils import type_util
 
 
+def _merge_dict(d, other):
+    for key, value in other.items():
+        _merge_item(d, key, value)
+
+
+def _merge_item(d, key, value):
+    item = d.get(key, None)
+    if type_util.is_dict(item) and type_util.is_dict(value):
+        _merge_dict(item, value)
+    else:
+        d[key] = value
+
+
 def merge(d, other, *args):
     others = [other] + list(args)
     for other in others:
-        for key, value in other.items():
-            src = d.get(key, None)
-            if type_util.is_dict(src) and type_util.is_dict(value):
-                merge(src, value)
-            else:
-                d[key] = value
+        _merge_dict(d, other)
     return d
diff --git a/benedict/dicts/keylist/keylist_dict.py b/benedict/dicts/keylist/keylist_dict.py
index d5e1794..55c56d2 100644
--- a/benedict/dicts/keylist/keylist_dict.py
+++ b/benedict/dicts/keylist/keylist_dict.py
@@ -15,7 +15,7 @@ class KeylistDict(dict):
         return super(KeylistDict, self).__contains__(key)
 
     def _contains_by_keys(self, keys):
-        parent, key, _ = keylist_util.get_item(self, keys)
+        parent, _, _ = keylist_util.get_item(self, keys)
         if type_util.is_dict(parent):
             return True
         elif type_util.is_list(parent):