确定字符串是否为Ruby保留字的内置方法？

\"next\".is_keyword?

class String
  def is_keyword?
    %w{__FILE__ __LINE__ alias and begin BEGIN break case class def defined? do else elsif end END ensure false for if in module next nil not or redo rescue retry return self super then true undef unless until when while yield}.include? self
  end
end
\"foobar\".is_keyword? # => false
\"for\".is_keyword? # => true

public

protected

string.keyword?

string.define

string.example