Splitting a list into its longest initially accepted list prefix and the remaining list suffix. On finite total lists with terminating predicate tests, the pair is a direct right fold: the true branch prepends to the first component; the false branch reconstructs the original tail by concatenating the two tail components and puts the current head before it. The richer pair result retains information that a direct dropped-list suffix list fold loses. Conventional recursive span is strict in the input spine; a literal pair of its two projections can expose an outer pair even on bottom, so partial-value equations require care.
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