docs: def make_default(spec):
      """Create an empty document that follows spec.  Any field with a default
      will take that value, required or not.  Required fields with no default
      will get a value of None.  If your default value does not match your
      type or otherwise customized Field class, this can create a spec that
      fails validation."""
      doc = {}
      for key, field in spec.iteritems():
          if field.default is not no_default:
              doc[key] = field.default
      return doc