REXML::Validation::RelaxNG

Let openmath2.rng be the file https://www.openmath.org/standard/om20-2019-07-01/openmath2.rng

If I try to validate using that file, I get the following error:

$ gem list |grep rexml
rexml (default: 3.2.3)
$ ruby -v
ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux-gnu]
$ irb -v
irb 1.2.1 (2019-12-24)
$ irb
irb(main):001:0> require "rexml/validation/relaxng"
=> true
irb(main):002:0> schema = File.new( "openmath2.rng" )
irb(main):003:0> validator = REXML::Validation::RelaxNG.new( schema )
Traceback (most recent call last):
        6: from /usr/bin/irb:23:in `<main>'
        5: from /usr/bin/irb:23:in `load'
        4: from /usr/lib/ruby/gems/2.7.0/gems/irb-1.2.1/exe/irb:11:in `<top (required)>'
        3: from (irb):3
        2: from (irb):3:in `new'
        1: from /usr/lib/ruby/2.7.0/rexml/validation/relaxng.rb:89:in `initialize'
NameError (uninitialized constant REXML::Validation::RelaxNG::AnyName)
irb(main):004:0>

Apart the AnyName, how exactly is used REXML::Validation::RelaxNG ? I didn't find the documentation about it

Start with REXML::Validation::RelaxNG and reproduce the failure using the supplied openmath2.rng example in Ruby 2.7 and IRB. Inspect relaxng.rb around line 89 and determine why the AnyName constant is unavailable. Done means the schema can be passed to RelaxNG without this NameError and the supported usage is documented.