module A1
  REACHABLE = true
  module B1
    NESTING = Module.nesting
    def self.reachable?
      REACHABLE
    end
  end
end

module A2
  REACHABLE = false
end
module A2::B2
  NESTING = Module.nesting
  def self.reachable?
    REACHABLE
  end
end


A1::B1::NESTING
A2::B2::NESTING

A1::B1.reachable?
A2::B2.reachable?