> + """Matrix Trace
> +
> + Represents the trace of a matrix expression.
> +
> + >>> from sympy import MatrixSymbol, Trace, eye
> + >>> A = MatrixSymbol('A', 3, 3)
> + >>> Trace(A)
> + Trace(A)
> +
> + >>> Trace(eye(3))
> + 3
> + """
> + is_Trace = True
> + def __new__(cls, mat):
> + if not mat.is_Matrix:
> + raise TypeError("input to Trace, %s, is not a matrix"%str(mat))
I think we shouldn't be close-minded about what Trace can handle. I would
suggest to allow other classes to define `_eval_trace` if it makes sense for
them, instead of restricting what kind of input Trace can have. This means
removing the check on `mat.is_Matrix` here. If `mat._eval_trace()` is defined,
we return that, if not, return an instance of `Trace`, as below. Then anybody
including the quantum folks will be able to use Trace without touching its
internals, by defining `_eval_trace` for their own classes. What do you think?
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