This is the one bit of logic that I altered.  The previous code only calculates and processes the diff if `isinstance(lhs, Expr) and isinstance(rhs, Expr)` is True.  I thought it would be better to try subtracting so that non-`Expr`s might make it through.  So I thought to catch `TypeError` in case `lhs - rhs` failed, and `AttributeError` in case the resulting diff does not have `.equals` or `.evalf` methods.  However, I just made a quick test, and doing only:

    try:
        diff = lhs - rhs
    except TypeError:
        pass
    [etc.]

manages to pass the test suite.  I can switch to this approach, or go back to the `isinstance` approach, however you prefer.