More docs, error messages, and tests
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index 4e93b15..8d24e23 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -365,9 +365,13 @@
The *fieldnames* are a single string with each fieldname separated by whitespace
and/or commas (for example 'x y' or 'x, y'). Alternatively, the *fieldnames*
- can be specified as a list of strings (such as ['x', 'y']). Any valid
- Python identifier may be used for a fieldname except for names starting and
- ending with double underscores.
+ can be specified as a list of strings (such as ['x', 'y']).
+
+ Any valid Python identifier may be used for a fieldname except for names
+ starting and ending with double underscores. Valid identifiers consist of
+ letters, digits, and underscores but do not start with a digit and cannot be
+ a :mod:`keyword` such as *class*, *for*, *return*, *global*, *pass*, *print*,
+ or *raise*.
If *verbose* is true, will print the class definition.