def append_opcode(opcode)
      raise ArgumentError, "Invalid opcode value." if opcode > 0xff || opcode < 0
      if opcode > 0 && opcode <= OP_PUSHDATA4
        raise ArgumentError, "Cannot add pushdata opcode without data"
      end
      @chunks << ScriptChunk.new(opcode.chr)
      return self
    end
def append_opcode(opcode)
      raise ArgumentError, "Invalid opcode value." if opcode > 0xff || opcode < 0
      if opcode > 0 && opcode <= OP_PUSHDATA4
        raise ArgumentError, "Cannot add pushdata opcode without data"
      end
      @chunks << ScriptChunk.new(opcode.chr)
      return self
    end
Modification
 ------------
 Appends a non-pushdata opcode to the script.