code: def modify_mempool(mempool, remove=0, add=0, verbose=False):
      
      for i in range(remove):
          popped = mempool.pop()
          if verbose: print("removed:", popped)
  
      for i in range(add):
          new_txid = _make_txid()
          mempool.append(new_txid)
          if verbose: print("added:", new_txid)
  
      return mempool
docs: Given a list of txids (mempool), add and remove some items to simulate
      an out of sync mempool.