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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def mk_definition(defn):
 """Instantiates a struct or SBP message specification from a parsed "AST" of a struct or message. Parameters defn : dict Returns A Definition or a specialization of a definition, like a Struct """

  assert len(defn) == 1
  identifier, contents = next(iter(defn.items()))
  fs = [mk_field(f) for f in contents.get('fields', [])]
  return sbp.resolve_type(sbp.Definition(identifier=identifier,
                                         sbp_id=contents.get('id', None),
                                         short_desc=contents.get('short_desc', None),
                                         desc=contents.get('desc', None),
                                         type_id=contents.get('type'),
                                         fields=fs,
                                         public=contents.get('public', True)))