gistfile1.txt

module M1
  refine String do
    def foo
      "M1#foo"
    end
  end
end

module M2
  refine String do
    def foo
      "M2#foo"
    end
  end
end

def bar(m, &block)
  block.using(m)
  Thread.pass
  block.call
end

using Proc::Refinements

100.times.map { |i|
  Thread.new(i.even? ? M1 : M2) { |m|
    bar(m) {
      p [m, "x".foo, Module.used_modules]
    }
  }
}.each(&:join)