It's totally thread safe, don't worry about it

gistfile1.rb

class TrueClass
  def self.setVal(val)
    $val = val
  end

  def <(n)
    $val < n
  end
end

class FalseClass
  def <(n)
    false
  end
end

class Fixnum
  alias_method :oldlt, :<

  def <(n)
    TrueClass.setVal(n)
    self.oldlt(n)
  end
end

puts 4 < 6 < 8