# this example does input validation, ensuring x is nonnegative: x = readInt() #PREMISES FOR ATTACHED PROOF, IF ANY: # True #PREMISES FOR NEXT LINE: if x < 0: #PREMISES FOR THEN-ARM: # (x < 0) x = 0 - x #PREMISES FOR ATTACHED PROOF, IF ANY: # (x == (0 - x_old)) # (x_old < 0) #PREMISES FOR NEXT LINE: else: #PREMISES FOR ELSE-ARM: # not (x < 0) pass #PREMISES FOR NEXT LINE: # not (x < 0) #PREMISES FOR NEXT LINE: # prove here that x >= 0