You're seeing just the function split_at, go back to Membrane.Payload module for more information.
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split_at(payload, at_pos)

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split_at(payload :: t(), at_pos :: pos_integer()) :: {t(), t()}

Splits the payload at given position (1st part has the size equal to at_pos argument)

at_pos has to be greater than 0 and smaller than the size of payload, otherwise an error is raised. This guarantees returned payloads are never empty.