YAML allows you to use anchors (&), similar to ID attributes in XML, and reference them later on (*). For example, if you have two profiles that are identical, you can tell YAML that one is an alias for the other:
development: &common db: oracle port: 8080 test: *common production: *common
If you have two elements that are almost identical, you can merge the values of one element into another (<<), for example:
development: &common db: hsql jdbc: hsqldb:mem:devdb test: <<: *common jdbc: hsqldb:file:testdb