<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_constantvalue(self):
 """ the constant pool index for this field, or None if this is not a contant field reference: http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.2 """
  # noqa

        buff = self.get_attribute("ConstantValue")
        if buff is None:
            return None

        with unpack(buff) as up:
            (cval_ref, ) = up.unpack_struct(_H)

        return cval_ref