9
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Why does

Circle[{0, 0}] /. List[_?NumericQ ..] :> Sequence[]

return

Circle[{0, 0}]

yet

Circle[{0, 0}] /. List[_?NumericQ ..] :> List

returns

Circle[List]

?

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12
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This is because the definition of Circle[] is Circle[{0, 0}]. As soon as {0, 0} is replaced by Sequence[], this definition kicks in, thus rendering the replacement useless.

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