code: def _signed_sub_overflow(state, a, b):
          
          sub = Operators.SEXTEND(a, 256, 512) - Operators.SEXTEND(b, 256, 512)
          cond = Operators.OR(sub < -(1 << 255), sub >= (1 << 255))
          return cond
docs: Sign extend the value to 512 bits and check the result can be represented
           in 256. Following there is a 32 bit excerpt of this condition:
          a  -  b   -80000000 -3fffffff -00000001 +00000000 +00000001 +3fffffff +7fffffff
          +80000000    False    False    False    False     True     True     True
          +c0000001    False    False    False    False    False    False     True
          +ffffffff    False    False    False    False    False    False    False
          +00000000     True    False    False    False    False    False    False
          +00000001     True    False    False    False    False    False    False
          +3fffffff     True    False    False    False    False    False    False
          +7fffffff     True     True     True    False    False    False    False