code: def merge_context(self, src=None):
          
          if src is None:
              # This condition will be true only if the server doesn't return a
              # txn context after a query or mutation.
              return
  
          if self._ctx.start_ts == 0:
              self._ctx.start_ts = src.start_ts
          elif self._ctx.start_ts != src.start_ts:
              # This condition should never be true.
              raise Exception('StartTs mismatch')
  
          self._ctx.keys.extend(src.keys)
          self._ctx.preds.extend(src.preds)
docs: Merges context from this instance with src.