This would probably be straightforward to do. For functions that accept matrix expressions and can auto-compute, I would probably want a MatrixType that is a union of e.g. Matrix, TransposedMatrix, MatrixInfixExpr, MatrixExpression, and MatrixIndexExpr.

If we add type annotations, I would want to run mypy in CI.

What do y'all think? I wouldn't mind seeing this added, but I'm unlikely to add it any time soon.