code: func (b *BlockGen) TxNonce(addr common.Address) uint64 {
  	if !b.statedb.Exist(addr) {
  		panic("account does not exist")
  	}
  	return b.statedb.GetNonce(addr)
  }
docs: // TxNonce returns the next valid transaction nonce for the
  // account at addr. It panics if the account does not exist.