code: def _translate_sext(self, oprnd1, oprnd2, oprnd3):
          
          assert oprnd1.size and oprnd3.size
  
          op1_var = self._translate_src_oprnd(oprnd1)
          op3_var, op3_var_constrs = self._translate_dst_oprnd(oprnd3)
  
          if oprnd3.size > oprnd1.size:
              result = smtfunction.sign_extend(op1_var, op3_var.size)
          elif oprnd3.size < oprnd1.size:
              raise Exception("Operands size mismatch.")
          else:
              result = op1_var
  
          return [op3_var == result] + op3_var_constrs
docs: Return a formula representation of a SEXT instruction.