code: def SIGNEXTEND(self, size, value):
          
          # FIXME maybe use Operators.SEXTEND
          testbit = Operators.ITEBV(256, size <= 31, size * 8 + 7, 257)
          result1 = (value | (TT256 - (1 << testbit)))
          result2 = (value & ((1 << testbit) - 1))
          result = Operators.ITEBV(256, (value & (1 << testbit)) != 0, result1, result2)
          return Operators.ITEBV(256, size <= 31, result, value)
docs: Extend length of two's complement signed integer