diff --git a/chatmastermind/tags.py b/chatmastermind/tags.py
index 28583a2..bfe5fd5 100644
--- a/chatmastermind/tags.py
+++ b/chatmastermind/tags.py
@@ -30,6 +30,67 @@ class Tag(str):
         return instance
 
 
+def delete_tags(tags: set[Tag], tags_delete: set[Tag]) -> set[Tag]:
+    """
+    Deletes the given tags and returns a new set.
+    """
+    return tags.difference(tags_delete)
+
+
+def add_tags(tags: set[Tag], tags_add: set[Tag]) -> set[Tag]:
+    """
+    Adds the given tags and returns a new set.
+    """
+    return set(sorted(tags | tags_add))
+
+
+def merge_tags(tags: set[Tag], tags_merge: list[set[Tag]]) -> set[Tag]:
+    """
+    Merges the tags in 'tags_merge' into the current one  and returns a new set.
+    """
+    for ts in tags_merge:
+        tags |= ts
+    return tags
+
+
+def rename_tags(tags: set[Tag], tags_rename: set[tuple[Tag, Tag]]) -> set[Tag]:
+    """
+    Renames the given tags and returns a new set. The first tuple element
+    is the old name, the second one is the new name.
+    """
+    for t in tags_rename:
+        if t[0] in tags:
+            tags.remove(t[0])
+            tags.add(t[1])
+    return set(sorted(tags))
+
+
+def match_tags(tags: set[Tag], tags_or: Optional[set[Tag]], tags_and: Optional[set[Tag]],
+               tags_not: Optional[set[Tag]]) -> bool:
+    """
+    Checks if the given set 'tags' matches the given tag requirements:
+    - 'tags_or' : matches if this TagLine contains ANY of those tags
+    - 'tags_and': matches if this TagLine contains ALL of those tags
+    - 'tags_not': matches if this TagLine contains NONE of those tags
+
+    Note that it's sufficient if 'tags' matches one of 'tags_or' or 'tags_and',
+    i. e. you can select a TagLine if it either contains one of the tags in 'tags_or'
+    or all of the tags in 'tags_and' but it must never contain any of the tags in
+    'tags_not'. If 'tags_or' and 'tags_and' are 'None', they match all tags (tag
+    exclusion is still done if 'tags_not' is not 'None').
+    """
+    required_tags_present = False
+    excluded_tags_missing = False
+    if ((tags_or is None and tags_and is None)
+       or (tags_or and any(tag in tags for tag in tags_or))   # noqa: W503
+       or (tags_and and all(tag in tags for tag in tags_and))):  # noqa: W503
+        required_tags_present = True
+    if ((tags_not is None)
+       or (not any(tag in tags for tag in tags_not))):  # noqa: W503
+        excluded_tags_missing = True
+    return required_tags_present and excluded_tags_missing
+
+
 class TagLine(str):
     """
     A line of tags. It starts with a prefix ('TAGS:'), followed by a list of tags,
@@ -71,37 +132,29 @@ class TagLine(str):
 
     def merge(self, taglines: set['TagLine']) -> 'TagLine':
         """
-        Merges the tags of all given taglines into the current one
-        and returns a new TagLine.
+        Merges the tags of all given taglines into the current one  and returns a new TagLine.
         """
-        merged_tags = self.tags()
-        for tl in taglines:
-            merged_tags |= tl.tags()
-        return self.from_set(set(sorted(merged_tags)))
+        tags_merge = [tl.tags() for tl in taglines]
+        return self.from_set(merge_tags(self.tags(), tags_merge))
 
-    def delete_tags(self, tags: set[Tag]) -> 'TagLine':
+    def delete_tags(self, tags_delete: set[Tag]) -> 'TagLine':
         """
         Deletes the given tags and returns a new TagLine.
         """
-        return self.from_set(self.tags().difference(tags))
+        return self.from_set(delete_tags(self.tags(), tags_delete))
 
-    def add_tags(self, tags: set[Tag]) -> 'TagLine':
+    def add_tags(self, tags_add: set[Tag]) -> 'TagLine':
         """
         Adds the given tags and returns a new TagLine.
         """
-        return self.from_set(set(sorted(self.tags() | tags)))
+        return self.from_set(add_tags(self.tags(), tags_add))
 
-    def rename_tags(self, tags: set[tuple[Tag, Tag]]) -> 'TagLine':
+    def rename_tags(self, tags_rename: set[tuple[Tag, Tag]]) -> 'TagLine':
         """
         Renames the given tags and returns a new TagLine. The first
         tuple element is the old name, the second one is the new name.
         """
-        new_tags = self.tags()
-        for t in tags:
-            if t[0] in new_tags:
-                new_tags.remove(t[0])
-                new_tags.add(t[1])
-        return self.from_set(set(sorted(new_tags)))
+        return self.from_set(rename_tags(self.tags(), tags_rename))
 
     def match_tags(self, tags_or: Optional[set[Tag]], tags_and: Optional[set[Tag]],
                    tags_not: Optional[set[Tag]]) -> bool:
@@ -117,14 +170,4 @@ class TagLine(str):
         'tags_not'. If 'tags_or' and 'tags_and' are 'None', they match all tags (tag
         exclusion is still done if 'tags_not' is not 'None').
         """
-        tag_set = self.tags()
-        required_tags_present = False
-        excluded_tags_missing = False
-        if ((tags_or is None and tags_and is None)
-           or (tags_or and any(tag in tag_set for tag in tags_or))   # noqa: W503
-           or (tags_and and all(tag in tag_set for tag in tags_and))):  # noqa: W503
-            required_tags_present = True
-        if ((tags_not is None)
-           or (not any(tag in tag_set for tag in tags_not))):  # noqa: W503
-            excluded_tags_missing = True
-        return required_tags_present and excluded_tags_missing
+        return match_tags(self.tags(), tags_or, tags_and, tags_not)