[luatex] How to get a \mid binary relation that grows in LuaTeX

[Ulrike Fischer]

Am Thu, 16 Nov 2017 10:28:56 +0100 schrieb Hans Hagen:

>> Can one query the number by name? So that
>> \Umiddle class \directlua{whatever(rel)}  gives now 5 and in the
>> next version 3?

> these numbers are documented part of traditional tex, so we use that 
> 'rel' number now (the internal ones are just that: internal)

My question was how to write code that works in a current texlive
(where you need \Umiddle class 5 to get relation spacing) *and* in a
future luatex where class 3 will be the correct value. 

Is querying the luatex version the only way? 

-- 
Ulrike Fischer 
http://www.troubleshooting-tex.de/