P0083r3 for basic_fields

Version: 90

Just wanted to know if a "merge" API for header would make a sense or not. I have a situation where I need to have some sort of default set of headers which can either be customised by a client code by providing another header. So, instead of doing the regular (and inefficient) way of iterating and performing a "set", using the "merge" operation of intrusive list would be better.

WDYT ?