code: def _signed_add_overflow(state, a, b):
          
          add = Operators.SEXTEND(a, 256, 512) + Operators.SEXTEND(b, 256, 512)
          cond = Operators.OR(add < -(1 << 255), add >= (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     True     True     True    False    False    False    False
          +c0000001     True    False    False    False    False    False    False
          +ffffffff     True    False    False    False    False    False    False
          +00000000    False    False    False    False    False    False    False
          +00000001    False    False    False    False    False    False     True
          +3fffffff    False    False    False    False    False    False     True
          +7fffffff    False    False    False    False     True     True     True