Both Uses

incidental
in
Childhood's End
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"Incidentally," said Rupert, changing the subject abruptly, "if you think this is a big job, you should see the commission Sullivan's got.†
Chpt 11

incidentally = of something that comes with something else, but is less important than it
Incidentally we've got a very fine symphony orchestra-probably among the world's half-dozen best.†
Chpt 15 *

Definitions:

(1) (incidental as in: an incidental expense)
something that comes with something else, but is less important than it
sometimes in a specialized sense, including:
- incidental expenses or when in context, just incidentals -- minor expenses not budgeted or not specified
- incidental music -- music in a play, television program, radio program, video game or some other form not primarily musical. (The term is less frequently applied to film music, with such music being referred to instead as the film score or soundtrack.)
(2) (meaning too rare to warrant focus)