`define-extended-language`+ overriding a `variable-not-otherwise-mentioned` with a `variable-not-in` fails.

[florence]

The program

#lang racket
(require redex/reduction-semantics)

(define-language L
  (v ::= GO WAIT x)
  (x ::= variable-not-otherwise-mentioned))

(define-extended-language L+ L
  (x ::= (variable-not-in WAIT)))

(redex-match? L+ x (term GO))

produces #f, despite x in L+ being specified to only exclude WAIT. It produces #t if the variable-not-in is replace with variable.