non-ascii characters not allowed in identifiers.

rules.org

@@ -134,13 +134,27 @@ This section contains guidelines on how to name objects in a program.
 Generally speaking, you should try to match the conventions used by whatever
 package you're working on.  But please try to always follow these rules:
 
+ - *Use only ASCII characters in identifier names* [<<ascii-identifiers>>]
+
+   This is what C++ calls the basic character set.  Specifically, identifiers
+   should use only the characters a-z, A-Z, 0-9, and underscore.
+
+   Handling of non-ascii characters is implementation-defined.
+   While many compilers can indeed handle extended (unicode) characters,
+   not all tools may process them correctly.  Some characters may not
+   display correctly, depending on a user's local installation.
+   Further, it is often not obvious how to type an arbitrary unicode
+   character that one sees displayed, especially since there exist
+   distinct characters that look very similar or identical.
+   
+
  - *Use prefix* =m_= *for private/protected data members of classes.*  [<<data-member-naming>>]
 
    Use a lowercase letter after the prefix =m_=.
 
    An exception for this is xAOD data classes, where the member names are
    exposed via ROOT for analysis.
-
+ 
 
  - *Do not start any other names with* =m_=. [<<m-prefix-reserved>>]
 
@@ -3006,6 +3020,9 @@ The comment includes the fact that it is the perpendicular distance.
 
 * Changes
 
+** Version 0.8
+ - Clarify that non-ascii characters should not be used in identifier names.
+
 ** Version 0.7
  - Minor cleanups and updates to take into acount that we now require C++17.
  - Use the =fallthough= attribute, not a comment.