def p2pkh?
      return false unless chunks.size == 5
      [OP_DUP, OP_HASH160, OP_EQUALVERIFY, OP_CHECKSIG] ==
          (chunks[0..1]+ chunks[3..4]).map(&:ord) && chunks[2].bytesize == 21
    end
def p2pkh?
      return false unless chunks.size == 5
      [OP_DUP, OP_HASH160, OP_EQUALVERIFY, OP_CHECKSIG] ==
          (chunks[0..1]+ chunks[3..4]).map(&:ord) && chunks[2].bytesize == 21
    end
whether this script is a P2PKH format script.