I'm using sympy matrix expressions for differentiation and I want to add 
some custom matrix functions and including spectral norm and eigenvectors. 
I also want to be able to differentiate expressions containing these 
functions.

The relevant documentation(
https://docs.sympy.org/latest/guides/custom-functions.html) says "functions 
that accept or return other kinds of objects than complex numbers should 
subclass another class, such as Boolean, MatrixExpr, Expr, or Basic." but 
only gives examples for Function.

Later it says "Subclasses of Expr that aren’t Function subclasses will need 
to define _eval_derivative()"

I have three questions relating to this

   1. When I subclass MatrixExpr what does _eval_derivative need to do?
   2. Do I need to define/hook/set/etc. anything else
   3. Does anyone have an example I could read?

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