# 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