code: def _make_plus_helper(obj, fields):
      
      new_obj = {}
  
      for key, value in obj.items():
          if key in fields or key.startswith('_'):
              # if there's a subschema apply it to a list or subdict
              if fields.get(key):
                  if isinstance(value, list):
                      value = [_make_plus_helper(item, fields[key])
                               for item in value]
              # assign the value (modified potentially) to the new_obj
              new_obj[key] = value
          else:
              # values not in the fields dict get a +
              new_obj['+%s' % key] = value
  
      return new_obj
docs: add a + prefix to any fields in obj that aren't in fields