ABC's

as long as there is  'A'  &  'B'

'A'  can look to  'B'  for difference

and  'B'  can look to  'A'  for difference

but if  'A' = 'B'  there might as well just be 'A'

or just  'B'

difference must be searched for

......no this is wrong

if 'A' knows  'B' = 'A'  and  'B'  knows  'A' = 'B'

they both become  'C'

and they both then see  'A'  and  'B'