[FOM] Question about Congruence

[Andrej Bauer]

hdeutsch at ilstu.edu wrote:
 > First, is the  simple fact about closer under equivalence well-known?

It is a mathematical triviality. As Vaughan Pratt said, it is evident 
rathern than well-known. It's the sort of simple thing that 
mathematicians wouldn't state as a separate theorem, except perhaps in a 
basic textbook that teaches students equivalence relations.

Andrej