Below is a dialogue between software developers, the first one used ChatGPT in his solution of code task and makes a comment,   the second one gives him reply on his solution. Write a reply that appropriately answers the comment.

### Comment:
 heres a partial description of why we need casts from gpt  not completely satisfying from a mypy perspective though ### Reply:
if we use typevarfoo, bounduniondataarray, dataset, then a function that calls separate methods e.g. .reindex, which is not a method on parent class of dataarray  dataset wont be unified. so if we have a function that takes as input variable t_dataarrayorset type variable and outputs t_dataarrayorset tyselect  from tabpe, this will fail, because the return type is expected as uniondataarray, dataset. thats why we need all these casts, and user code would have the same problem. but when we use typevarfoo, dataarray, dataset, then subclasses dont work. and we have three quite similar type definitions, which arent fully compatible with each other. im not sure its possible to resolve these. ive had a search, but not much came up. possibly wed need to write a protocol...?