Macro definitions should be parsed first

PyYaml seems to parse yaml files in an undefined order. Assume the following:

defs:
- !defmacro
  name: test_one
  params: {}
  definition:
    replaced1: ok
- !defmacro
  name: test_two
  params: {}
  definition:
    replaced2: ok
    replaced3: ok

testnode: !multimacro
    - name: test_one
    - name: test_two

This is currently not possible, because !multimacro is executed before the defs-node. So the !defmacro statements come to late for the testnode to be affected.

This, however, would work:

defs:
- !defmacro
  name: test_one
  params: {}
  definition:
    replaced1: ok
- !defmacro
  name: test_two
  params: {}
  definition:
    replaced2: ok
    replaced3: ok

testnode:
    obl: !multimacro
    - name: test_one
    - name: test_two

How can we ensure that the node "defs" is parsed first?