Is this not possible within the native whois client?
One reason it’s difficult is that there isn’t a 1:1 mapping between ASN and AS-SET.
For example, AS786 uses AS-JANETUS, AS-JANETPLUS, and AS-JANETEURO (the names are largely historic, JANETUS is for transit, JANETEURO is for the pan-European R&E network, JANETPLUS is for all other bi-lateral peerings).
To get the correct one for our peering with you, you’d need to query AS786 and look at the relevant export: line, or talk to the NOC when bringing up a peering (although for the bulk of peerings it’s AS-JANETPLUS, as documented in Peering DB).
Cheers, Rob
