title: Ruamel yaml formatting of dumped nested lists
body: <p>I would like to dump a dictionary with components that are nested lists, where each row of the list is on its own line. I also want to maintain dictionary order.</p>
  
  <p>Here's a MWE:</p>
  
  <pre><code>import sys
  
  from ruamel import yaml
  from ruamel.yaml import YAML
  
  d = {'b':1,
       'a':[[1, 2],[3, 4]]}
  
  # desired output:
  # b: 1
  # a:
  # - [1, 2]
  # - [3, 4]
  
  print()
  yaml.dump(d, sys.stdout)
  print('\n')
  YAML().dump(d, sys.stdout)
  </code></pre>
  
  <p>And here's what I actually get:</p>
  
  <pre><code>a:
  - [1, 2]
  - [3, 4]
  b: 1
  
  
  b: 1
  a:
  - - 1
    - 2
  - - 3
    - 4
  </code></pre>
  
  <p>The first method has the nested-list formatting I want, but loses dictionary order. (Not a surprise since I'm not using a round-trip dumper) The second method manages to maintain order, but loses my desired nested list formatting. Any time I use a round-trip dumper, I've lost the nice nested list formatting.</p>
  
  <p>Any tips here?</p>