Sort of related, I find the docs hard to follow, and I think this might
be because of the score lowering function reads characters rather than
being a command prefix.  I can see why it does this; it seems like they
are parameters, but it means the command docs don't explain the subcases:

  gnus-summary-lower-score is an interactive autoloaded compiled Lisp
  function in `gnus-score.el'.

  (gnus-summary-lower-score &optional SCORE SYMP)

  Make a score entry based on the current article.
  The user will be prompted for header to score on, match type,
  permanence, and the string to be used.  The numerical prefix will
  be used as SCORE.  A symbolic prefix of `a' (the SYMP parameter)
  says to use the `all.SCORE' file for the command instead of the
  current score file.

  [back]


Even reading the source, the behavior of followup is not entirely clear.