<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def gen_sext(src, dst):
 """Return a SEXT instruction. """

        assert src.size <= dst.size

        empty_reg = ReilEmptyOperand()

        return ReilBuilder.build(ReilMnemonic.SEXT, src, empty_reg, dst)