at the core, the Next() method moves another searcher forward
and checks each hit to see if it also satisfies the phrase
constraints. the current implementation has 4 nested for loops.
these nested loops make it harder read (indentation) and harder
to reason about (complexity).
this refactor does not remove any loops, it simply moves some
of the inner loops into separate methods so that one can
more easily reason about the parts separately.
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