What is the term for saying something like A is B,and B is A?

Chiasmus is the classical term for examples of 'cross-over phrasing'.

How 'classical'

Nice to see you ... to see you, nice.

was is another matter.


Mathematically, the statement "A is B and B is A" postulates an equivalence of A and B, i.e. that they can be used interchangeably (as opposed to one being a subset of the other).