def height
      return nil if header.version < 2
      coinbase_tx = transactions[0]
      return nil unless coinbase_tx.coinbase_tx?
      buf = StringIO.new(coinbase_tx.inputs[0].script_sig.to_payload)
      len = Bitcoin.unpack_var_int_from_io(buf)
      buf.read(len).reverse.bth.to_i(16)
    end
def height
      return nil if header.version < 2
      coinbase_tx = transactions[0]
      return nil unless coinbase_tx.coinbase_tx?
      buf = StringIO.new(coinbase_tx.inputs[0].script_sig.to_payload)
      len = Bitcoin.unpack_var_int_from_io(buf)
      buf.read(len).reverse.bth.to_i(16)
    end
return this block height. block height is included in coinbase.
 if block version under 1, height does not include in coinbase, so return nil.