新手问题 关于 Rails 的数据库配置文件 database.yml 当中，Yaml 的表达式解释的问题，查阅了一些资料还是没有了解
# SQLite version 3.x
# gem install sqlite3
#
# Ensure the SQLite 3 gem is defined in your Gemfile
# gem 'sqlite3'
#
default: &default
adapter: sqlite3
pool: 5
timeout: 5000
development:
<<: *default
database: db/development.sqlite3
# Warning: The database defined as "test" will be erased and
# re-generated from your development database when you run "rake".
# Do not set this db to the same as development or production.
test:
<<: *default
database: db/test.sqlite3
production:
<<: *default
database: db/production.sqlite3
其中&default定义了引动点，*defalut进行了引用，不明白的地方在于符号<<:的作用是什么？
