> Let me know if I'm misunderstanding the proposal.  Are you proposing a non-standard ASDL extension to that won't work with existing ASDL parsers? 

No, I am not proposing any modification to the ASDL parsers nor the ASDL definition language.

I am proposing a modification to the internal data structures we use internally to carry sequences of ASDL types. Currently, asdl_seq is a type that holds an array of void* pointers and the user is supposed to cast the individual items to the correct types, which is error prone. I am proposing to add a new field to our internal structures that allows the user to have some certainty over what these sequences contain.