Q: OED quotations for "flying

[Joel S. Berson]

The OED has for sense 4.a, "That passes or travels swiftly, that
passes by rapidly or rushes along; rapid. flying leap: a running jump
[etc.]" the earliest citation as 1658.

But it then has for sense 4.b, "esp. in flying post, a post
travelling by relays of horses, an express [etc.]." the earliest
citation as 1548.

Why shouldn't the 1548 quotation for 4.b, which is a subclass of
sense 4.a, be shown in 4.a itself?  Particularly when 4.a includes
some compounds, and the 4.b quotation is (much) earlier.

Joel

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