docs: def suck_out_variations_only(reporters):
      """Builds a dictionary of variations to canonical reporters.
  
      The dictionary takes the form of:
          {
           "A. 2d": ["A.2d"],
           ...
           "P.R.": ["Pen. & W.", "P.R.R.", "P."],
          }
  
      In other words, it's a dictionary that maps each variation to a list of
      reporters that it could be possibly referring to.
      """
      variations_out = {}
      for reporter_key, data_list in reporters.items():
          # For each reporter key...
          for data in data_list:
              # For each book it maps to...
              for variation_key, variation_value in data["variations"].items():
                  try:
                      variations_list = variations_out[variation_key]
                      if variation_value not in variations_list:
                          variations_list.append(variation_value)
                  except KeyError:
                      # The item wasn't there; add it.
                      variations_out[variation_key] = [variation_value]
  
      return variations_out