tsearch2 questions

1. What is the advantage of the tsearch2() trigger? Why can't I write my
own trigger which does approximately:

  UPDATE manuscript set manuscript_vector =
    setweight(to_tsvector(manuscript_genre), 'A') ||
    setweight(to_tsvector(manuscript_title), 'B') ||
    to_tsvector(manuscript_abstract);

2. Is there a way to know in advance the maximum return value of the
rank function? I have lots of other information to include in the
goodness-of-match score besides the fulltext match rank so I would
prefer a tsearch2 rank score between 0 and 1. Do I need to write my own
rank function?

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