def public_key_hash_script?
      return false if @chunks.size != 5
      return @chunks[0].opcode == OP_DUP &&
             @chunks[1].opcode == OP_HASH160 &&
             @chunks[2].size == 21 &&
             @chunks[3].opcode == OP_EQUALVERIFY &&
             @chunks[4].opcode == OP_CHECKSIG
    end
def public_key_hash_script?
      return false if @chunks.size != 5
      return @chunks[0].opcode == OP_DUP &&
             @chunks[1].opcode == OP_HASH160 &&
             @chunks[2].size == 21 &&
             @chunks[3].opcode == OP_EQUALVERIFY &&
             @chunks[4].opcode == OP_CHECKSIG
    end
Returns true if the script is a P2PKH script:
 "OP_DUP OP_HASH160 <20-byte hash> OP_EQUALVERIFY OP_CHECKSIG"