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Raymond,

Thanks for the attention to detail. But could you help me
understand what it means that "the whole approach is
misguided, and the use case itself is suspect[?]" The
customization use case? The approach to customization via
the mapping argument?

In other words, I understood the mapping argument to already
be a customization mechanism; otherwise, the methods could
use kws only. So wouldn't a 'key' argument be *more*
complicated, introducing additional customization semantics
instead of working with what's already there?

My concern, though, is that by using _multimap instead of
update, we limit the customizations one may perform from the
outside. Why this limitation? Performance? Ok. So then it's
a matter of adjudicating a performance/feature trade-off?