xor on strings

[dsavitsk]

maybe this isn't xor, but i would like something that will smush strings ...
that is, something that would compare two strings position by position and
return the higher (or lower i suppose) value piece in each position, i.e.

>>> fun('00', 'AA')
'AA'

>>> fun('00', 'A')
'A0'

>>> fun('00', '')
'00'

i understand that this is easy to write, but i was looking for a builtin.
does it exist?

-d