```ruby
module BuildPluginHelpers
  def build_start_paypal_plugin(account_id = nil)
    if account_id.nil?
      plugin = build_plugin(::Killbill::PaypalExpress::PaymentPlugin, 'paypal_express')
      start_plugin plugin
    else
      account_line = "  - :account_id: #{account_id}"
      Dir.mktmpdir do |dir|
        file = File.new(File.join(dir, 'paypal_express.yml'), 'w+')
        open('paypal_express.yml', 'r') do |origin_file|
          line_num = 0
          origin_file.each_line do |line|
            file.puts(account_line) if line_num == 1
            file.puts(line)
            line_num += 1
          end
        end
        file.close
        plugin = build_plugin(::Killbill::PaypalExpress::PaymentPlugin, 'paypal_express', File.dirname(file))
        start_plugin plugin
      end
    end
  end

  def start_plugin(plugin)
    svcs = plugin.kb_apis.proxied_services
    svcs[:payment_api] = PaypalExpressJavaPaymentApi.new(plugin)
    plugin.kb_apis = ::Killbill::Plugin::KillbillApi.new('paypal_express', svcs)
    plugin.start_plugin
    plugin
  end
end
```

Revise indent format for yml loading
user
```ruby
module BuildPluginHelpers
  def build_start_paypal_plugin(account_id = nil)
    if account_id.nil?
      plugin = build_plugin(::Killbill::PaypalExpress::PaymentPlugin, 'paypal_express')
      start_plugin plugin
    else
      account_line = "  - :account_id: #{account_id}"
      Dir.mktmpdir do |dir|
        file = File.new(File.join(dir, 'paypal_express.yml'), 'w+')
        open('paypal_express.yml', 'r') do |origin_file|
          line_num = 0
          indent_end = false
          origin_file.each_line do |line|
            # insert account id line in the second line (paypal_express.yml starting from the first line)
            file.puts(account_line) if line_num == 1
            # indent the line between account_id and database setting
            need_indent = line_num == 0 ? false : true
            indent_end = true if line.strip == ':database:'
            line = line.strip.indent(4) if need_indent && !indent_end
            file.puts(line)
            line_num += 1
          end
        end
        file.close
        plugin = build_plugin(::Killbill::PaypalExpress::PaymentPlugin, 'paypal_express', File.dirname(file))
        start_plugin plugin
      end
    end
  end

  def start_plugin(plugin)
    svcs = plugin.kb_apis.proxied_services
    svcs[:payment_api] = PaypalExpressJavaPaymentApi.new(plugin)
    plugin.kb_apis = ::Killbill::Plugin::KillbillApi.new('paypal_express', svcs)
    plugin.start_plugin
    plugin
  end
end
```
assistant