code: def append_change_trust_op(self, asset_code, asset_issuer, limit=None, source=None):
          
          asset = Asset(asset_code, asset_issuer)
          op = operation.ChangeTrust(asset, limit, source)
          return self.append_op(op)
docs: Append a :class:`ChangeTrust <stellar_base.operation.ChangeTrust>`
          operation to the list of operations.
  
          :param str asset_issuer: The issuer address for the asset.
          :param str asset_code: The asset code for the asset.
          :param str limit: The limit of the new trustline.
          :param str source: The source address to add the trustline to.
          :return: This builder instance.