post-N246 Read- and Write-Option variable_names/1 (Was: DCG restrictions on the left-hand side [Novacore])

UWN is testing Prolog systems that don't exist anymore:
Jekejeke 1.5.0
https://www.complang.tuwien.ac.at/ulrich/iso-prolog/variable_names#73
a) There is no more publicitly available Jekejeke
    Prolog anymore, I think I wrote UWN already in the
    past about that.
b) There is only Dogelog Player, which has a 100%
    Prolog written read/write predicates including
    the variable_name/1 option. Source code is here,
    you also find GIT access etc to the Prolog system:
Quelltexte und Archive des Dogelog Spieler.
https://www.xlog.ch/izytab/doclet/docs/07_spec/02_dogelog.html
c) There is an internal version of Jekejeke Prolog,
    which is way after 1.5.0, with version 1.7.0, but
    it also uses the Dogelog Payer 100% Prolog
    written read/write, so no need to test.
Mild Shock schrieb:

Guess how many newer Prolog systems allow this here:
 ?- [user].
p;q --> r.
    true.
 On the other hand SWI-Prolog disallows it:
 ?- [user].
p;q --> r.
 ERROR: user://1:29:
ERROR:    No permission to define dcg_nonterminal `p;q'