Add to an XXX comment.
diff --git a/Lib/io.py b/Lib/io.py
index 4ac4e45..fdf1299 100644
--- a/Lib/io.py
+++ b/Lib/io.py
@@ -601,7 +601,8 @@
# XXX This doesn't work; _MemoryIOMixin's write() and truncate()
# methods assume the buffer is mutable. Simply redefining those
# to use slice concatenation will make it awfully slow (in fact,
- # quadratic in the number of write() calls).
+ # quadratic in the number of write() calls). Also, there are no
+ # readline() and readlines() methods. Etc., etc.
def __init__(self, inital_string=None):
buffer = ""