def define_context(context_name)
      rule_sets.fetch(context_name) do |context_name|
        rule_sets[context_name] = RuleSet.new
      end

      self
    end
def define_context(context_name)
      rule_sets.fetch(context_name) do |context_name|
        rule_sets[context_name] = RuleSet.new
      end

      self
    end
Initialize and assign a RuleSet for the named context

 @param [Symbol] context_name
   the name of the context to be defined. noop if already defined

 @return [self]

 @api private