class Capybara::Selector::Definition

def expression(type, allowed_filters, &block)
  if block
    @expressions[type] = block
    allowed_filters = parameter_names(block) if allowed_filters.empty?
    allowed_filters.flatten.each do |ef|
      expression_filters[ef] = Capybara::Selector::Filters::IdentityExpressionFilter.new(ef)
    end
  end
  @expressions[type]
end