REXML::Validation::RelaxNG

[MarcoCostantini]

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