Sympy expression evaluation using custom objects

[Kiran Chandra]

I am new to Sympy and I am trying to evaluate an expression using custom objects.
To explain it a bit more, let us say I have a class Custom and I have add and multiply operations defined for the objects of Custom.

Now if I have an expression like x + y, I would like to use 2 objects of Custom and the custom add function.

How do I get started to do this?

thanks
Kiran